Replace symmetric difference list operation with something more biologist-friendly

A mathematical symmetric difference will keep members of an odd amount of lists, which when used with >2 lists means 3, 5, etc.

This is neither intuitive nor useful in a biological context. What we want instead is to keep only members of 1 list, or described in set operations, `(union of all sets) minus (pairwise intersection of all sets)`.

Based on feedback from Gos.