Merge pull request #3559 from RosettaCommons/vmullig/residue_index_description_issue
Resolve an issue with assignment of -1 to an unsigned int.
Tweaking ResidueIndexDescription, which was invalidly assigning -1 to an unsigned int. Note that this was *not* causing any breakage here, but it's a design pattern that confused the ICC compiler (which threw a warning) and might have confused future developers. It's probably best to avoid this in order to prevent *human* error.