Doc string clarification request

[davidlmobley]

In HybridTopologyFactory, there's an option which seems important but for which we can't fully understand the doc string. Specifically:

 interpolate_old_and_new_14s : bool, default False
            Whether to turn off interactions for new exceptions (not just
            1,4s) at lambda = 0 and old exceptions at lambda = 1; if False,
            they are present in the nonbonded force.

Issues: a) What does this have to do with 1,4s? (Implied by the name) b) Why does the doc string say it doesn't apply to just 1,4s? c) What are "old" vs "new"? Are we talking about the two states in the perturbation, e.g. A vs B state? d) Why is this referring to exceptions? e) What's the interpolation implied by the name? ("interpolate" vs "whether to turn off" seem to be two very different things)

Our best guess as to what this is doing/claiming to do is to control whether nonbonded interactions are turned off for dummy atoms. We ran into a case where our dummy atoms were forcing the attached physical/real atoms to stay in a particularly odd conformation (relative to the conformation expected if the dummy atoms are not present) and turning this option to True resulted in the physical atoms returning to the "expected" conformation. My student's interpretation of this was that this is because the nonbonded interactions are otherwise left on (if we don't set this to True) and that results in weird forces/conformations. However, then I had all the questions above and she couldn't answer them.

I'm guessing the fix here is to clarify the doc string and possibly change the name of the option, if it doesn't specifally have to do with interpolation and 1-4s?

cc @Yoshanuikabundi