Reorganize rules section, add a section on times/durations for accumulated and similar variables (#139)

## Description
Add rules for variables requiring references to dates, times, and durations. The existing [ISO 8601](https://en.wikipedia.org/wiki/ISO_8601) standard is used, though because of Standard Name rules we must use lowercase rather than uppercase letters for the time, date, and duration identifiers, unlike the capital letters specified in the standard. Two new suffixes are also added (``_over_T`` and ``_reset_every_T``) to supplement the existing ``_since_T``.

In addition, I broke the "ESM Standard Name Rules" section into a few subsections for better organization.

## Issues
Resolves #130

Author: mkavulich

StandardNamesRules.rst

@@ -21,6 +21,9 @@ for use with Earth System Models. It describes the
 ESM Standard Name Rules
 ========================
 
+Constructing names
+------------------
+
 #. Standard names should be identical to those from the latest version
    of the `Climate and Forecast (CF) metadata
    conventions <https://cfconventions.org/standard-names.html>`_ unless
@@ -64,10 +67,34 @@ ESM Standard Name Rules
    and ``air_temperature``. When multiple transformations are present, a more detailed description
    should be provided in the ``description`` field to prevent any possible ambiguity.
 
+Variable scope
+--------------
+
 #. Variables are current and instantaneous unless specified. Variables that are not
    current (e.g., previous timestep) or non-instantaneous (e.g., accumulated values)
    should have qualifiers in the standard name to describe what they represent.
 
+#. For accumulated variables, or variables representing a change over some period of time, the
+   following suffixes are available":
+
+   * ``over_[time]`` indicates an accumulation or other change over the previous duration/time
+   * ``reset_every_[date/time]`` an accumulation or other change reset every set duration/time
+     since the start of the simulation
+   * ``since_[date/time]`` indicates an accumulation or other change since a given date/time.
+
+   Dates, times, and durations should follow the `ISO 8601 <https://en.wikipedia.org/wiki/ISO_8601>`
+   international standard, modified only to use lowercase rather than uppercase letters. Note that
+   the standard is slightly different for dates and times vs durations. For example:
+
+   * ``accumulated_precipitation_over_pt3h`` accumulated precipitation over the last 3 hours
+   * ``accumulated_precipitation_over_p1dt12h`` accumulated precipitation over the last 1 day 12 hours
+   * ``accumulated_precipitation_reset_every_pt1h`` accumulated precipitation reset every 1 hour
+   * ``accumulated_precipitation_reset_every_p1y`` accumulated precipitation reset every 1 year
+   * ``accumulated_precipitation_reset_every_p2dt12h`` accumulated precipitation reset every 2 days, 12 hours
+   * ``accumulated_precipitation_since_20230522t120000`` accumulated precipitation since May 22, 2023 at 12:00
+   * ``accumulated_precipitation_since_20251225`` accumulated precipitation since December 25, 2025
+   * ``accumulated_precipitation_since_t00`` accumulated precipitation since 00:00:00 (midnight)
+
 #. By default (when not specified otherwise), variables are grid means or centers
    (defined by the host). If a variable is defined at a different physical location,
    a qualifier should be used to denote this. For example, to specify the vertical
@@ -92,6 +119,26 @@ ESM Standard Name Rules
    ``[variable]_at_top_interfaces`` is defined on ``n`` points, and
    ``[variable]_at_bottom_interfaces`` is defined on ``n`` points.
 
+#. If possible, qualifiers should be limited in order to allow for a wide
+   applicability of the variable. In other words, don't qualify with ``_for_specific_context``
+   unless a variable could not conceivably be used outside of the more
+   narrowly-defined context or a variable without the scope-narrowing qualifiers
+   already exists and cannot be reused.
+
+   **Discouraged:** upward_virtual_potential_temperature_flux_for_mellor_yamada_janjic_surface_layer_scheme
+
+   **Preferred:** upward_virtual_potential_temperature_flux
+
+#. If there are two identical quantities from different schemes/processes that
+   need to be kept apart, suitable qualifiers are added to the names of the processes.
+   If one process is already established and more common than the other, then it is
+   sufficient to only prefix the new process with a suitable qualifier. Example:
+   ``due_to_convective_GWD`` and ``due_to_convective_whole_atmosphere_GWD``
+   as discussed in https://github.com/ESCOMP/ESMStandardNames/issues/79.
+
+Terminology
+-----------
+
 #. By default, *mixing_ratio* refers to mass mixing ratios. The description should
    explicitly specify that it refers to the *mass* mixing ratio.
    Mass mixing ratios should contain information regarding
@@ -131,23 +178,6 @@ ESM Standard Name Rules
    of clouds the variable represents (e.g. *ice_cloud* if only including glaciated clouds, or
    *cloud_at_500hPa* if only including clouds that exist at 500 hPa).
 
-#. If possible, qualifiers should be limited in order to allow for a wide
-   applicability of the variable. In other words, don't qualify with ``_for_specific_context``
-   unless a variable could not conceivably be used outside of the more
-   narrowly-defined context or a variable without the scope-narrowing qualifiers
-   already exists and cannot be reused.
-
-   **Discouraged:** upward_virtual_potential_temperature_flux_for_mellor_yamada_janjic_surface_layer_scheme
-
-   **Preferred:** upward_virtual_potential_temperature_flux
-
-#. If there are two identical quantities from different schemes/processes that
-   need to be kept apart, suitable qualifiers are added to the names of the processes.
-   If one process is already established and more common than the other, then it is
-   sufficient to only prefix the new process with a suitable qualifier. Example:
-   ``due_to_convective_GWD`` and ``due_to_convenctive_whole_atmosphere_GWD``
-   as discussed in https://github.com/ESCOMP/ESMStandardNames/issues/79.
-
 #. Spell out acronyms unless they are obvious to a vast majority of
    scientists/developers who may come across them. A list of currently-used
    aliases is below. Whenever such an alias exist, use the alias in the
@@ -389,6 +419,8 @@ Suffixes
 | on_previous_timestep
 | ``N`` _timesteps_back
 | since_ ``T``
+| over_ ``T``
+| reset_every_ ``T``
 
 Computational
 -------------