Revisiting force training keyword

[Alireza Khorshidi]

There are different coefficients related to force training: "train_forces", "force_coefficient", "force_rmse", and "force_maxresid". We need to think about which one can be removed, which one is the major keyword, and different scenarios for the user.

[andrew_peterson]

Yes -- I think the user still has to specify in multiple places how to turn off force training. This needs to be made simpler.

[andrew_peterson]

Btw, it might be three places. The force_coefficient in LossFunction, the force_rmse in LossFunction's convergence dictionary, and the train_forces keyword in calc.train!

[andrew_peterson]

This should be taken care of in commit 81f4508. However, I almost certainly broke the tensorflow module in this commit, and I don't have a working installation at the moment, so I expect a new issue to arise while we get that figured out!

[andrew_peterson]

Issue #59 was marked as a duplicate of this issue.