add STR variants

[tcrombie]

Add bi-allelic genotype matrix for STR variants. A first pass would be to filter STRs to those that are bi-allelic and wait on incorporation of multi-allelic STRs.

Because STRs are not all bi-allelic we will eventually need to devise a way of incorporating multi-allelic sites. I have not investigated possibilities for doing statistical tests using a multi - allelic matrix.