allocate_regimes_from_shifts allocate a number (from 0 to the number
of shifts) to each node, corresponding to its regime : all nodes below shift
i are numbered by i.
allocate_regimes_from_shifts(phylo, shifts_edges)a phylogenetic tree, class phylo.
edges were the shifts are.
Vector of size (ntaxa + Nnode) of the regimes of each node and tip.