Add SOAP forcefields

[WardLT]

Use SOAP, as implemented in DScribe, as features for a GPR regression model.

Still need to

• Use GPU for training
• Adjust hyperparameters
• Build EnergyModel wrapper
• Test for example problems

[WardLT]

There's still more to do with SOAP, but let's merge what we have now. The forces predicted by SOAP are very poor compared to those from MBTR, and I suspect this has to do with how the model's trained or the choice of hyperparameters rather than a mistake in the software (see unit test comparing numerical to analytic forces).