Add function for getting entity type

[cthoyt]

From @bgyori

Classification of entities: I think this classification can be automatically inferred from membership so I'm not sure if it should be something that is curated/modifiable or something that is displayed on e.g., the website but is determined automatically based on an entry's children.

It would be good to have a function in the famplex package API (and therefore the FamplexGraph class) that returns the entity type given a FamPlex identifier. This could also be used to improve the static site export