compute_actualization_matrix_ultrametric computes a squares p*nedges bloc diagonal
matrix of the (I_p - exp(-A * (h - t_pa(j))))_j node.
compute_actualization_matrix_ultrametric(tree, selection.strength,
times_shared = compute_times_ca(tree))a phylogenetic tree.
the selection strength of the process.
a matrix, result of function compute_times_ca.
Matrix of size p*nedges
Carreful: the root is not taken into account in this function.