Create feature classes

[DaniBodor]

Build a parent class for DeepRankFeature and then use this to create child classes for each independent feature, that contain an add_feature function.
This allows for much better control when building the graph as well as makes a "guideline" for custom features.

Might be too much effort to convert current structure, but would def be an improvement.

[DaniBodor]

This will probably be useful once we start making new features for new data types (such as nucleic acids, etc).

[github-actions]

This issue is stale because it has been open for 30 days with no activity.