Add windows option to genealogical_nearest_neighbours

[jeromekelleher]

#184 fixed a bug in the definition of the GNN statistic. Need to update the API to add an extra parameter treewise=False to allow this to be returned treewise.

[jeromekelleher]

More generally, this should be defined in windows like other tree stats.

[jeromekelleher]

We can push this back to 0.2.1 because after #313 is merged the default behaviour for windows=None is to strip the first dimension (which is what this is doing now). So, when updating we need to add an option windows=None which follows the same semantics as the other windowed tree stats.

[hyanwong]

I think this may be rolled in to #683