Extension for nitrogen molecule.py and molecule.pxd

- adding function isNitrogen() to confirm that an atom is a nitrogen
atom
- updated and renamed the functions identifying lone pair - radical
electron shift resonance isomers
- added functions identifying resonance isomers with nitrogen atoms
having two double bonds (N4dd) and nitrogen atoms having one triple and
one single bond (N4ts)
- removed isRadical() in generateResonanceIsomers() because it is again
used inside getAdjacentResonanceIsomers() and
getLonePairRadicalIsomers() and is not required for
getN4dd_N4tsResonanceIsomers()