[IMP-dev] Loading multiple molecules from a pdb file

We need to clean up what happens when multiple molecules are loaded from a PDB. My current favorite is to - make the current read_pdb only support loading a single protein (chain) from the first model of a PDB file and return it as a Hierarchy with the root being a PROTEIN. It can spit out warnings if there are other molecules in there.

- add a read_model_from_pdb which loads all molecules from a given model in the PDB (defaulting to the first) and which returns an array of Hierarchy decorators each of which has a root of either a PROTEIN or a MOLECULE (or some other name to specify a molecule that is not a protein or nucleic acid).

Thoughts?