problems with `MTX.SubGModule`

[ThomasBreuer]

The following problems are present in GAP 4.12 and in the GAP master branch.
They have been observed by @fieker.

gap> G:= SymmetricGroup(3);;
gap> M:= PermutationGModule( G, GF(2) );;
gap> mat:= [ [ 1, 1, 1 ], [ 0, 1, 1 ] ] * Z(2);;
gap> orig:= StructuralCopy( mat );;
gap> MTX.SubGModule( M, mat );;
gap> mat = orig;    # the function call changes the 2nd argument
false
gap> MTX.SubGModule( M, Immutable( orig ) );;    # the 2nd argument must be mutable
Error, no method found! For debugging hints type ?Recovery from NoMethodFound
Error, no 1st choice method found for `TriangulizeMat' on 1 arguments at [...]

I think that MTX.SubGModule is not allowed to change its 2nd argument,
and that also an immutable 2nd argument should be allowed.

The easiest solution would be to call TriangulizedMat instead of TriangulizeMat.

[ThomasBreuer]

Pull request #5477 fixes just the reported problem.
It would be good to check that all (documented) MTX functions have the property that either they do not change their arguments or their documentation states that arguments may be changed.

[fingolfin]

While it might be good to "check all MTX functions" for the absence of similar bugs, I don't feel this is useful to keep around as an issue. After all, the same could be said for almost any GAP function.