Create a random ranked phylogenySource:
Create a random ranked phylogeny using the coalescent method.
This is implementation of a ranked coalescent algorithm described in Collienne (2021).
n tips of a tree, all with rank 0, randomly select two tips and merge them
into a new node with rank 1, and add the new node to remaining tips. In next iteration, assign
rank 2 and so on, until only a single node, the root, remains.