[1731][eval][infra] refactoring eval readers

[mtar]

Description

This PR cleans up the evaluation readers. Comments and doctrings are added/extended. Changes in functions.

• Reader abstract base class
• MergeReader in merge_reader.py

Issue Number

Closes #1731

Is this PR a draft? Mark it as draft.

Checklist before asking for review

• I have performed a self-review of my code
• My changes comply with basic sanity checks:
  • I have fixed formatting issues with ./scripts/actions.sh lint
  • I have run unit tests with ./scripts/actions.sh unit-test
  • I have documented my code and I have updated the docstrings.
  • I have added unit tests, if relevant
• I have tried my changes with data and code:
  • I have run the integration tests with ./scripts/actions.sh integration-test
  • (bigger changes) I have run a full training and I have written in the comment the run_id(s): launch-slurm.py --time 60
  • (bigger changes and experiments) I have shared a hegdedoc in the github issue with all the configurations and runs for this experiments
• I have informed and aligned with people impacted by my change:
  • for config changes: the MatterMost channels and/or a design doc
  • for changes of dependencies: the MatterMost software development channel

[mtar]

The PR has been split file-wise into

• [1888][eval] Refactor Reader class #1889
• [1890][eval] Move MergeReader to own module #1892
• [1893][eval] csvreader cleanup #1906
• [1907][eval] clean up wegen_reader.py #1911