ENH: use gap characters in <seqs>.get_identical_seqs()

[GavinHuttley]

The SequenceCollection.get_identical_sets() (and AlignmentI.get_identical_sets()) methods do not account for gaps, i.e. "ACG-" and "ACGT" are considered distinct. Add a "mask_gap" argument to the methods for both classes.

Should also split the methods into two private sub-methods that handle whether either mask_degen or mask_gap are set.