update.allocate_regimes_from_shifts computes the regime of a daughter
node, knowing the regime at the parent node and the vector of shifts positions
update.allocate_regimes_from_shifts(edgeNbr, ancestral, shifts_edges, ...): Number of the edge considered
: regime of the parent node
positions on edges
regime of the daughter node.
This function is used in function allocate_regimes_from_shifts and is
designed to furnish function recursionDown with the right structure of
data.