Widen declaration for many matrix related functions

[fingolfin]

Also remove some now redundant methods.

Before:

gap> m:=RandomMat(256, 256, GF(1000000007));;
gap> mat:=Matrix(IsZmodnZMatrixRep, GF(1000000007), m);;
gap> RankMat(mat)=256; time;
true
170
gap> DeterminantMat(mat); time;
ZmodpZObj( 873487643, 1000000007 )
23982

After:

gap> m:=RandomMat(256, 256, GF(1000000007));;
gap> mat:=Matrix(IsZmodnZMatrixRep, GF(1000000007), m);;
gap> RankMat(mat)=256; time;
true
161
gap> DeterminantMat(mat); time;
ZmodpZObj( 873487643, 1000000007 )
1949

As discussed on #6088

[wilfwilson]

Seems fine to me.

The diffs in the manual examples are harmless, as far as I can tell (the relevant groups are equal, they're just given by different generating sets).

[wilfwilson]

There's still one pesky harmless failing manual example 🙁