[IMP-dev] reading dna

the IMP pdb reader now reads (at least some) dna. Ben should probably look through the code to make sure I am not abusing modeller somehow.

Also IMP.read_pdb now returns a MolecularHierarchyDecorator rather than a Particle. This seems more reasonable to me as you need the former in order to do anything useful with it.

Hopefully now files are missing.