ExplicitBitVect: Add +operator to concatenate two ExplicitBitVects
MolOps: Add function to get the number of atoms with a distinct property
ChemReactions: Extented functionality of chemical reactions

- Module to generate reaction fingerprints: structural FP for screening,
  difference FP for similarity search
- Add some utility functions to simplify handling of reactions
- Add new functionality to include agents in the reactions, some adaptions
  to parsers and writers had to be done
- Supports intra-bond breaks in products now
- Molecules with rxn role (atom property) can be parse as reaction now
PGSQL cartridge: Add new reaction functionalities to cartridge

- Supports reaction type
- Input reaction as SMILES, SMARTS or ctab
- Supports GiST-index substucture/superstructure scans
- Supports reaction fingerprints