Finalizes Mutantrace Datum Update

Finalizes https://github.com/tgstation/-tg-station/pull/3609. Squashes several commits into one, and fixes some minor conflicts... again...

This should be the final pull request.
Shouldn't it be if(!(pref_species in species_list))? Wouldn't the current one check if species_list contains Null? Same for all other  unencapsulated use of the X in List statements in the code.