Update Fst and Tajima's D functions to use grouping convention

[jeromekelleher]

#100 added functions Fst and Tajimas_D which take two datasets as input. However, in #224 we discuss how the convention should be to use a grouping variable within the dataset to do this. Once the convention has been established, update these functions to use it.

Related: #239, #238, #224, #226, #225

[tomwhite]

Fixed in #260