IsomorphismGModule( module1, module2 )
This function tests the G-modules module1 and module2 for
isomorphism. Both G-modules must be defined over the same field with
the same number of defining matrices, and at least one of them must be
known to be irreducible (that is, IsIrreducible(module) has returned
true). Otherwise the function will exit with an error. If they are
not isomorphic, then false is returned. If they are isomorphic, then a
d times d matrix is returned (where d is the dimension of the
modules) whose rows give the images of the standard basis vectors of
module1 in an isomorphism to module2.
The algorithm is described in [5].
GAP 3.4.4