Remove GWInterfaceBase, GWInterface, and co.

[bdvllrs]

GWInterfaceBase, GWInterface, and co, were useless abstractions of encoders and decoders.

Now, GWModule additionaly takes the domain_mods as parameter.
Moreover, instead of requiring gw_intefaces it now needs to mapping: gw_encoders and gw_decoders which are torch.nn.Modules that encode or decode the unimodal representation to and from the GW representation.

See updated docs/shimmer_basics.md and examples to see how to update existing code.