Add DigraphRandomWalk

[mtorpey]

This adds a new operation that executes a random walk on a graph, starting at a vertex, and having the desired length.

I haven't done doc or tests yet, but does this look like the right sort of thing?

Output is the same as DigraphPath: a pair [v,a] where

• v is the ordered list of vertices in the path;
• a is the list of indices of edges followed from each vertex (helps distinguish edges in multidigraphs).

gap> D := Digraph([[1, 2], [3], [2, 4], [1], [2, 4]]);                
<immutable digraph with 5 vertices, 8 edges>
gap> DigraphRandomWalk(D, 1, 4);
[ [ 1, 2, 3, 2, 3 ], [ 2, 1, 1, 1 ] ]

If we reach a sink, we cut the path short, so the final length might be less than requested.

[flsmith]

As discussed, Michael plans to rename this length to something less expectation-loaded which forces users to read the documentation, so that surprises from shorter-length paths being returned are minimised.

[mtorpey]

@flsmith: This is ready for merging (as far as I can tell!)

[flsmith]

Looks good!