Fix raising matrix group elements to negative powers

[ThomasBreuer]

If the underlying matrix is a ZZMatrix then negative exponents are not allowed.

(addresses a problem mentioned in #5550)

Note that the situation is a bit tricky:

julia> G = GL(2, ZZ);

julia> x = gen(G, 1);

julia> x^-1;  # this worked already before this pull request

julia> n = -1;

julia> x^n;
ERROR: DomainError with -1:
Exponent must be non-negative
Stacktrace:
 [1] ^(x::ZZMatrix, y::Int64)
   @ Nemo ~/.julia/packages/Nemo/kdloy/src/flint/fmpz_mat.jl:325
 [2] ^(x::MatrixGroupElem{ZZRingElem, ZZMatrix}, n::Int64)
   @ Oscar ~/.julia/packages/Oscar/S0B41/src/Groups/matrices/MatGrp.jl:437
 [3] top-level scope
   @ REPL[6]:1

The reason for that is some literal_pow magic from AbstractAlgebra.

julia> @which x^n
^(x::MatrixGroupElem, n::Int64)
     @ Oscar ~/.julia/packages/Oscar/S0B41/src/Groups/matrices/MatGrp.jl:437

julia> @which x^-1
literal_pow(::typeof(^), g::GroupElem, ::Val{-1})
     @ AbstractAlgebra ~/.julia/packages/AbstractAlgebra/L8iQ0/src/Groups.jl:228

We might do the same for ZZMatrix.

[fingolfin]

This literal_pow magic is something Julia does in general...

[thofma]

Would fixing powering for ZZMatrix also be an option?

[ThomasBreuer]

Would fixing powering for ZZMatrix also be an option?

It is not clear for me that "fixing" is the right term, my understanding was that the DomainError is intentional.

If we want to change this then this is fine.

(The change belongs to Nemo.jl.
Also the literal_pow magic for exponent -1 could then be added there.)

[thofma]

Hm, since we don't error here:

julia> ZZ(-1)^-2
1

I think we should also not error in the matrix case. I'll adjust it.