Aromaticity detection work in progress.
Implemented Huckel's rule for determining aromaticity.

* Removed addition of H atoms (and their bonds) when reading input
  molecules.
Aromaticity determination happens as soon as a ring is added.

* Earlier, it was performed as part of a ring's normalisation, but
  it would not affect normalised atom numbering in that case.  This
  is because of normalisation of atoms happening sooner than that of
  rings.  Ring aromaticity bears an effect on the unsaturation state
  of its atoms.
Molecule normalisation work in progress.

* Atom (re)ordering is now in place.
* Normalised IDs are assigned to atoms.
* Rings are normalised accordingly, too.
Fixed an error in normalisation.  Resolved issue #14.

* Heuristically, reordering of atoms and assignment of normalised
  IDs is being done thrice.  This may need further testing and
  tuning later.
Bonds forming linking chains are now identified.
Added a special case for sulfur in aromaticity detection.

* The specific case is that of a sulfur with two exocyclic double
  bonds, with both its ring bonds being single.