Another attempt at propagating symmetry breaking constraint

Fix a bug where node 7 didn't get color 6 removed as in the following trace:

0: {0}
1: {0}
2: {0, 1}
3: {0, 1, 2}
4: {0, 1, 2, 3}
5: {0, 2, 3, 4}
6: {1}
7: {0, 1, 2, 3, 4, 5, 6}
Guard against infeasible solutions causing NullPointerExceptions
Ensure that final solutions are feasible
Another other attempt at propagating the symmetry breaking constraint

The invariant on cumm[v] needn't be true until the function returns. We only
need cumm[v-1] to behave as expected when the function begins.
Readability improvements to symmetry breaking constraint
Make the DegreeSortedGraph sort descending order of degree
Don't double add edges to DegreeSortedGraph
Add Javadoc header to DegreeSortedGraph