Multipopulation two-site statistics

[lkirk]

We'd like to implement two-population versions of the statistics we've implemented thus far: $D$, $D^{\prime}$, $D^{2}$, $D_{z}$, $\pi_{2}$, $r$, $r^{2}$

This means that we'll strictly require two sample sets, comparing the A locus from sample set 1 and the B locus from sample set 2.

There is a lot of prior art and the way we've structured two-locus statistics, this should be relatively straightforward.

Are there any other stats we want to implement @apragsdale?