Integrating Magnet Challenge 2 models

[wkirgsn]

The second Magnet Challenge has concluded and many H-field predicting sequence models were developed. With the current magnethub API, these models should be compatible.
A unifying system for the key-based model selection should be elaborated.

[gituser789]

may the default should link to the magnetchallenge 1 models, for existing code using mag-net-hub as a dependency (to not break this code)

[wkirgsn]

I envision the integration of the MC2 models to be shipped by a SequenceModel from sequence.py, while the loss models from MC1 stay wrapped with LossModel from loss.py. That would be a hard separation and the naming should explain what the difference is. Please look at the PR for a first draft of this new wrapper