Optimal proper coloring, chromatic number, fractional chromatic number

[dstahlke]

Would you be open to a pull request implementing an algorithm for chromatic number and another for fractional chromatic number? The former uses PicoSAT and the latter would use some linear programming solver, perhaps GLPK via JuMP. The chromatic number implementation is here, if you want to take a quick peek: https://gist.github.com/dstahlke/8e4fd40fa845e792ff7cd3f6b4ccb124

[gdalle]

Sorry for the delay, but that would be great @dstahlke!

[dstahlke]

Done. https://github.com/gdalle/GraphsOptim.jl/pull/12