Import mechanism from Python entry_points

[maarten-ic]

Prototype import mechanism for discovering yMMSL configuration provided by Python entry_points.

See iterorganization/IMAS-Streams#12 on how a python package may use this:

• Declare entry-points in pyproject.toml:

  [project.entry-points."ymmsl.path"]
  "imas_streams/data_source.ymmsl" = "imas_streams.muscle3_datasource:DATA_SOURCE"

• The entry point should resolve to a string attribute containing a valid yMMSL configuration. See https://setuptools.pypa.io/en/latest/userguide/entry_point.html for further details on the entry point mechanism.

TODO:

• Discuss proposed solution
• Decide whether YMMSL_PATH or entry points take precedence
• Use importlib_metadata backport for python < 3.10
• Clean up code and add docstrings
• Document the mechanism