matrix_of_possibles is used in function update.enumerate_parsimony
to compute, from the list of possible matrices for the daughters, the matrix for
the node (a group for the parent being fixed).
matrix_of_possibles(matrices)a list of matrices with ndaughters entries.
Matrix of all possible regimes for the subtree bellow node parent.
This function select all possible combinations of rows from all the daughters, and
merge then into one using function merge_complementary_vectors.