Use Incremental algorithm for computing tree ranks on tree sequences

[daniel-goldstein]

One can compute the combinatorial rank of a tree with Tree.rank (see here). Currently, this is only used for very small trees, but if a user wanted to count unique topologies in a tree sequence with larger numbers of samples, t.rank() for t in ts.trees() would be very wasteful and slow. Calculating tree ranks is done leaves->root so, like typical incremental algorithms, only an upward traversal after an SPR is needed to adjust the rank during a tree transition. The simple incremental algorithm to generate per-tree ranks in a tree sequence would follow almost exactly the same approach as counting species tree topologies, except:

• the per node state would be a RankTree instead of a TopologyCounter
• per-root state would not be merged since multiply-rooted trees cannot be ranked, it would likely have to return a list of ranks that maps to the roots of the current tree