turn CPESet2_3.compare_wfns into a generator

This way some comparisons can be short-circuited for faster matches...
Merge branch 'optimize-compare-2.3' of https://github.com/TauPan/cpe into TauPan-optimize-compare-2.3
Merge branch 'TauPan-optimize-compare-2.3' into develop. Closes #9