Loss choice: instantiate with hydra

[remtav]

See similar issue #246

Currently, choosing a loss in model_choice.py calls an init_.py that makes use of a series of disorganized if-else statements.
Choosing a model can be much more straight forward by using config files for each relevant loss and pointing to the model class directly in this config file. See hydra's documentation on Class instantiation.

[remtav]

closed with #256