Add option to window the popgen plots over n trees

[duncanMR]

Gertjan Bisschop suggested that we add another option for windowing the Popgen tab plots, such that each window covers n trees for a given n. At the moment, we can either choose a fixed number of windows or have one window per tree (i.e. n =1).

[benjeffery]

Tsqc is just exposing the tskit options here, I don't think multiple trees is an option. Could be though if someone wanted to implement it.

[jeromekelleher]

Would be easy enough, something like

windows = ts.breakpoints(as_array=True)[::k]

would pull out every kth breakpoint (but would need some special handing of the trailing values) I think?

[benjeffery]

Popgen tab has been shelved so closing for now.