Replace apricot with submodlib

[ghost]

Fixes #16
submodlib is now used for the CRAIG strategy/dataloader as well as the submodular strategy/dataloader. Please let me know if you have any feedback!

Notes:

• I am not sure if sum redundancy (a submodular function implemented in apricot) has an analogue in submodlib, so it is disabled as an option for now.
• It doesn't seem like submodularselectionstrategy.py is used in the corresponding dataloader. This may be a good opportunity to refactor, so that behavior is consistent between the two.
• Any existing code that specifies the "optimizer" (greedy algorithm) used by apricot will break, since the names used by submodlib are different than those used by apricot (e.g. 'LazyGreedy' instead of 'Lazy'). This includes configs that use this option.

[krishnatejakk]

@mikespencer720 Thanks a lot for your help. I have started the code review. Will push the code if there are no issues.