Merge all OT mapping methods in one class

[rflamary]

Should we merge OT mapping methods?

it seems like the use of entropic regularization or group lasso could be added easily as a parameter to the same class. We should probably keep LinearOTMapping separate because it is very different in term of mapping but the OTMapping, EntropicOTmapping and Grouplasso could be merged in one class in my opinion and it would make SKADA less OT biased ;).

Another question is if we should move them in _ot.py . on this one I'm a little more circumspect since those are mapping method and they probably belong with CORAL...