Feature rename repertoires

[MartinMao2023]

Related issues: #227 [Unify the naming of repertoires]
Unified the naming of repertoires used by the emitters and containers.

This PR introduces:

• removed the Repertoire class along with the "repertoire.py" file
• Renamed the GARepertoire to GAPopulation, this also means renaming "ga_repertoire.py" to "ga_population.py".
• In files where the Repertoire class was imported, changed to import GAPopulation instead.
• In "dcrl_emitter.py" particularly, used MapElitesRepertoire to replace the Repertoire class.

Checks

• a clear description of the PR has been added
• sufficient tests have been written
• example notebook have been added to the repo
• all example notebooks have been verified to execute without errors
• clean docstrings and comments have been written
• relevant section have been added to the documentation
• if any issue/observation has been discovered, a new issue has been opened
• the source branch is develop if this PR introduces a new feature

Future improvements

Nothing yet.