find_actualized_shift_values computes the values of the shifts their
positions, and the mean values at the tips.
Warning : this function does not check for consistency. Please make sure that the
shifts positions and the mean values are compatible.
find_shift_values(shifts_edges, T_tree_ac, vect_Y, p, ntaxa)a vector of positions of shifts on the tree.
matrix of incidence of the tree, result of function
incidence.matrix.
the vector of values of the means at the tips.
vector, with first entry the values at the root, and other entries the values of the shifts.
This function uses qr.solve for rectangular linear system solving.