Get all possible contributor pairs from a 2 person mixture
contributor_pairs(mixture)A list of integer vectors. The k'th element in the list is an integer vector with the alleles in the mixture at locus k.
A contrib_pairs object that is a unordered list of pairs.
Note, that contributor order is disregarded so that each contributor pair is
only present once (and not twice as would be the case if taking order into
consideration). See example usage at rank_contributor_pairs.