Improve clique finding for digraphs with isolated vertices #23
Description
For a digraph with isolated vertices, the automorphism group tends to be large - since the isolated vertices can be permuted at will. Therefore calculations involving the automorphism group of such digraphs may take longer. However, for cliques, an isolated vertex is only ever in a trivial (maximal) clique - so we really don't need to compute with these at all. (And they're probably not relevant to other computations either). Therefore the clique finder should pre-process a digraph to remove its isolated vertices using ReducedDigraph.