Categorize atoms into solvent, ligand, etc. categories

The categories and criteria are:

solvent

The most numerous "small" (10 atom or less) single-residue chain that isn't a singleton atom of atomic number 9 or more (florine or heavier) and that occurs at least 10 times.

ions

Singleton atoms not categorized as solvent.

ligand

Chains smaller than 1/4 the longest chain of the model that aren't solvent or ions.

main

Remainder of chains.