Optimise validation.py

[JoeZiminski]

Currently the validation is not well optimised and loops over the same list multiple times in various places. In general this may not be a problem as the lists arn't going to be huge (e.g. not more than 100 subjects I'd imagine). Nonetheless things might get slow for large projects, also it in some cases makes the logic hard to follow.

See validate_names_against_project() docstring, this comment and docstring of validate_list_of_names() for discussions.