instead of using the property map interface.
A nice side-effect is that the wart of having to use property maps to loop over bonds or atom neighbors
is now gone.
This potentially breaks lots of client C++ code.
Conformer.cpp: make sure we don't ask for space for zero atoms.
Kekulize.cpp: don't complain about marked bonds if we aren't removing markers.
molopstest.cpp: bad smiles corrected in one test.exe
ROMol.cpp, RWMol.cpp: extra preconditions added (should have been there all along)
Wrap/MolOps.cpp: docs for replacesubstructs.
