OEAddColorAtoms now has an optional third argument to cause precalculation
and caching of the self color. When re-using molecules in multiple
comparisons (for example NxN studies), use OEAddColorAtoms once and then
OEBestOverlay and OEColorOverlap will use the pre-stored color atoms and
self color to speed up the calculations.
Added a set of functions to manipulate cached self color.