Added copy in DigraphAllChordlessCycles

[MeikeWeiss]

I had the problem that after calling DigraphAllChordlessCycles for a graph the output of DigraphAllSimpleCircuits for this graph was wrong. The output included a list of nodes that did not define a valid cycle. In my opinion the problem was that SetDigraphVertexLabels(digraph, Reversed(DigraphDegeneracyOrdering(digraph))) changed the given digraph. So now I added a call so that we only consider a copy of the input graph.
This solved my problem, but I'm not sure this is the best way.

The graph I was having problems with was the following:

 g:=Digraph([ [ 2, 3, 7 ], [ 1, 4, 8 ], [ 1, 4, 9 ], [ 2, 3, 10 ], [ 6, 7, 9 ], [ 5, 8, 10 ], [ 8, 5, 1 ], [ 7, 6, 2 ], [ 5, 10, 3 ], [ 6, 9, 4 ] ]);

[james-d-mitchell]

I'm not sure this resolves the issue, if D is mutable, then DigraphRemoveAllMultipleEdges(D) changes D in-place (I think), same with DigraphRemoveLoops and DigraphSymmetricClosure. So when D is mutable we copy it once here.

If D is immutable, then each of these 3 functions copies D, and so too does DigraphCopySameMutability so we make 4 copies of D.

If you put the DigraphCopySameMutability as the inner most function call, then I think would work, but would also still be suboptimal (in terms of the number of copies).

Can't we just instead do

digraph := DigraphMutableCopy(D); # 1 copy
# the next line changes digraph in-place and does not touch the original input D
# also there are no copies. 
DigraphSymmetricClosure(DigraphRemoveLoops(DigraphRemoveAllMultipleEdges(digraph))); 
MakeImmutable(digraph); # make digraph immutable, o/w it can't store attributes.

[james-d-mitchell]

Thanks for the PR @MeikeWeiss, please consider my comment, and let me know if that also works. If it does, then please add your previously failing example as a test case too.

[MeikeWeiss]

Thank you for your comment! It seems to work.