explain slightly better what test_cxx_mpz_mat does
explain slightly better what test_cxx_mpz_mat does

mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs,mancoast/cado-nfs