somewhat optimized version from Roger. 
get the return vector the right size.
note: this still is not thread safe.

git-svn-id: svn://svn.code.sf.net/p/rdkit/code/trunk@2415 a5045d30-4826-4484-9c7f-6e55a06ddc77
use the boost::flyweight pattern to get thread-safe initialization

git-svn-id: svn://svn.code.sf.net/p/rdkit/code/trunk@2416 a5045d30-4826-4484-9c7f-6e55a06ddc77