Plot heatmap of polytomy density over space and time

[duncanMR]

Given our interest in how polytomies affect tsdate performance, it would be very helpful to be able to create a heatmap of polytomy density in bins across space and time. This would be particularly useful in conjuction with the mutations plot, since we can test whether the dense bands in mutation times correlate with regions of high polytomy density.

We briefly discussed the implementation of this plot in the tsdate meeting last week: @jeromekelleher had an idea for how to iterate through the trees and keep track of node arity in time bins. Nate Pope pointed out that this algorithm is similar to the coalescent rate calculator, so we might be able to repurpose that code.