For phylo2vec vectors, get_common_ancestor returns the most recent common ancestor (MRCA) of two nodes in a Phylo2Vec vector node1 and node2 can be leaf nodes (0 to n_leaves) or internal nodes (n_leaves to 2*(n_leaves-1)) (similar to get_common_ancestor in ETE (Python)).

We should implement a similar function for matrices. Optionally, we could return the height of the MRCA for both functions.

Node depth was handled in #171, node height will be treated in a separate issue.