Use Python entry points for runtime discovery of plugins to improve modularity and robustness

[lbianchi-lbl]

This FOQUS Enhancement Proposal (FEP?) shows how switching to Python entry points for discovering plugins at runtime can lead to improvements for FOQUS in terms of reliability, code duplication, and maintanability.

If implemented, the proposed solution would directly or indirectly resolve the following existing issues and PRs:

• Review code executed at import time #955
• Possible improvements to plugin functionality #908
• Improve plug-in visibility, documentation and configuration #818
• classes for tracking optional dependencies #1032
• Declaring FOQUS Optional Dependencies #1013