Make BespokeFit continue running if there are connectivity rearrangements in only some fragments

[j-wags]

In #193, one of the goals that wasn't accomplished due to lack of time was having BespokeFit continue running in the event where some fragments have proton migrations, but not others. Since the torsion-parameter-to-fragment-to-torsiondrive mapping isn't one-to-one, there may be some complexity in figuring out which torsion parameters to drop under various scenarios.