cache and load registry

[guybuk]

Currently users can't use their own data categories (and cache/load methods). Instead, we:

1. deprecate the DataCategory enum (since enums can't be expanded on)
2. replace separate cache and load methods with a DataIO class. The idea is that saving and loading is very similar in essence (e.g. np.load/np.save) and with a class, users don't have to use very similar code in two different sections of the repo.
3. Allow registering DataIO classes