absolute_continuity

[IshiguroYoshihiro]

Motivation for this change

This PR introduces a definition of absolute continuity.
Absolute continuity is a key condition in the second fundamental theorem of calculus for the Lebesgue integral: the increment of an absolutely continuous function over an interval is given by the Lebesgue integral of its almost-everywhere derivative.

This PR relates to #1892.
This notion will be used in several lemmas in the theory of Lebesgue integration, such as the change-of-variables theorem for the Lebesgue integral.

Checklist

• added corresponding entries in CHANGELOG_UNRELEASED.md

• added corresponding documentation in the headers

Reference: How to document

Merge policy

As a rule of thumb:

• PRs with several commits that make sense individually and that
  all compile are preferentially merged into master.
• PRs with disorganized commits are very likely to be squash-rebased.

Reminder to reviewers

• Read this Checklist
• Put a milestone if possible
• Check labels