Identical copies of the same method in AbstractAtomContainerSet, but using IMolecule, solving an exception complaining about IMoleculeSet to contain only IMolecule [only for cdk-1.4.x]

Change-Id: I8d3555efe5e6be9b5685d346b623aec42bb3c4b1
Signed-off-by: John May <john.wilkinsonmay@gmail.com>