Change "LIG" to UNL (unknown ligand).

Updated to match PDB documentation..
http://www.wwpdb.org/documentation/format32/sect4.html

Hat tip to Roger Sayle.
Be careful with P aromaticity. Add explicit rule for PR#898.

TODO: revisit aromaticity calculation for P atoms.
Add minor refinements to ConnectTheDots for bond detection.

As suggested by Roger Sayle, add "fail quickly" as we check the cutoff.
If x^2 or x^2+y^2 is beyond the cutoff, then the full distance is too.
Fix PR#897. Aromatic nH is OK in 5-membered rings.

6-membered pyridinium nH -> nH+, but 5-membered rings are OK.