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.