`core.Ring` work in progress.
`core.Ring` work in progress.

* `core.Molecule` can now compute inter-atomic distances.
Normalisation code begins.

* Ring detection code work in progress.
`lib.ExhaustiveRingDetector` has single ring detection logic.
Ring detector is now injected during normalisation of a molecule.
Minor class rearrangement.

* Moved `core.AbstractGroup` to `util`.
* Moved `core.FunctionalGroups` to `lib`.