Adds sum formula function to PG cartridge.

	* Function signature equivalent to Python interface's CalcMolFormula().
	* Only defined for Postgres 9.x but should work on 8.x too.
Merge branch 'pg-add-mol_fmla' of https://github.com/janholstjensen/rdkit into janholstjensen-pg-add-mol_fmla
rename fmla->formula