Resolve potential packmol bug or at least communicate to their developers

@nividic - after you're back, please follow up on the potential packmol bug from https://github.com/MobleyLab/SolvationToolkit/blob/master/solvationtoolkit/solvated_mixtures.py#L467 - i.e. raise with the developers there to get them to fix. Even if it has trouble converging, it should never add TOO MANY molecules.

Thanks.
