Update popgen stats to compute for all variants

[jeromekelleher]

#100 added support for some basic popgen stats. However, these all currently return a single value, which contradicts the idiom that we are establishing of "every function returns an updated dataset" (#103). I think the default behaviour of these functions should be to return the full array of values in the updated dataset, for example, in the diversity function we'd add a new variable stats_diversity which is assigned the value of pi that we have computed (before summing).

This would include the Fst (#225) statistic, so is part of #232 and #226.

[tomwhite]

This was fixed in #303 and documented in #361