Add invariant: chromatic_sum

[math1um]

This was discussed at SIAM DM'18.

The chromatic_sum is the smallest sum of vertex labels, when the vertices are labeled with positive integers and adjacent labels are labeled with different integers ("colors").

See: https://en.wikipedia.org/wiki/Sum_coloring

This is intractable, and at least as hard as chromatic_number. A naive algorithm is: find the chromatic number k, then try all possible labels of the vertices with 1,2..,k, check if its a proper coloring, compute the sum, and store the current minimum.

[yirkajk]

That naive algorithm is in fact not slow enough 😞. According to the wiki, achieving the unique chromatic sum may require more labels than the chromatic number. So, we would have to check even more than 1,2,..,k.