Allow SASA penalty term to be parameterized as well?

[jchodera]

I just realized we may also want to allow the SASA penalty term to be fit as well.

Ideally, we'd just compare the performance of the GB part alone, but OpenMM doesn't actually compute the SASA the same way that Amber does, so there may be utility in allowing the penalty term to be fit as well.

This isn't high-priority, though.