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.
The pair of functions (OEColorAtomsToString and
OEStringToColorAtoms) added in v1.7.1 have been
renamed to more appropriate names. The new functions are
OECompressColorAtoms and
OEUncompressColorAtoms.