Added clique_number, independence_number

[dstahlke]

These simple implementations use maximal_cliques as the computational engine. Perhaps there is a more efficient way, but it would be great to at least have these implemented.

[gdalle]

Thanks for the PR, sorry for the late review

[dstahlke]

I've pushed an update that hopefully addresses these concerns. (I rebased to master - hopefully that doesn't make the diffs too messy.)

After making a proper unit test for independent sets, I found I had to convert AbstractGraph to SimpleGraph in order to call complement. Performance shouldn't be an issue since this data copy is negligible in comparison to the cost of the maximal_cliques call. But I don't know if there is some other reason this would be problematic.

@traitfn function maximum_independent_set(
    g::AG::(!IsDirected)
) where {T,AG<:AbstractGraph{T}}
    # Convert to SimpleGraph first because `complement` doesn't accept AbstractGraph.
    return maximum_clique(complement(SimpleGraph(g)))

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f6f8db6) 97.28% compared to head (470df1c) 97.23%.
Report is 7 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #155      +/-   ##
==========================================
- Coverage   97.28%   97.23%   -0.06%     
==========================================
  Files         115      115              
  Lines        6789     6790       +1     
==========================================
- Hits         6605     6602       -3     
- Misses        184      188       +4     

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[gdalle]

I would add a warning to the docstring pointing to the conversion, but otherwise I'd say it's okay