Expose base classes and hide metaclasses

[Yoshanuikabundi]

This is a bit of a drive-by but I figured it was easier to write this PR than explain it in an issue :/

Previously the abstract base classes for post-processing layers and GCN stacks were private. I realised this when I started to write up some hints on how to write your own and found that the base classes weren't documented. The registry metaclasses, however, are public and documented. I think this could lead to some major confusion, as the actual functionality you need to implement isn't in the metaclass, its in the base class. I think the metaclasses are really implementation details for the string encoding stuff, and they're the metaclasses of the base classes so inheritors shouldn't need to use them explicitly anyway.

So I've...

Changes made in this Pull Request:

• Made AtomFeatureMeta, BondFeatureMeta, PostprocessLayerMeta and GCNStackMeta private by prepending them with underscores and removing them from __all__ as appropriate
• Made BaseGCNStack public by adding it to the gcn module re-exports
• Made Feature public by adding it to the feature module re-exports
• Updated the GNNModel docstring to use the base classes instead of the metaclasses.

I think this was just an oversight but if it's intended then no worries!

PR Checklist

• Tests?
• Docs?
• CHANGELOG updated?
• Issue raised/referenced?

[Yoshanuikabundi]

Wow that went badly Nevermind, sorted out my error :)

[lilyminium]

Seems like a good idea, thanks!