Patch from Roger to enable Mol file support for atom classes

With this patch, MOL files now read in atom class information (3rd from final column in atom block) by default, and write it out if the "-xa" option is specified. This is the same as with the SMILES format.

Test case roundtripping SMI->MOL->SMI:
obabel -:[CH4:1] -omol -xa | obabel -imol -osmi -xa
[CH4:1]