Function: bnrgaloisapply
Section: number_fields
C-Name: bnrgaloisapply
Prototype: GGG
Help: bnrgaloisapply(bnr, mat, H): apply the automorphism given by its matrix
 mat to the cngruence subgroup H given as a HNF matrix. The matrix mat can be
 computed with bnrgaloismatrix
Doc: apply the automorphism given by its matrix \var{mat} to the congruence
 subgroup $H$ given as a HNF matrix.
 The matrix \var{mat} can be computed with \tet{bnrgaloismatrix}.