[IMP-dev] Making PDBSelectors Objects

I'd like to make PDBSelectors IMP::Objects. The change would not affect python code. For C++ users, PDBSelectors would have to be created with new instead of on the stack. So in general, a call that was read_pdb(name, CAlphaPDBSelector()) would become read_pdb(name, new CAlphaPDBSelector())

The advantages are - we could now provide AndPDBSelector and OrPDBSelector which work in python - there aren't annoying corner cases which cause problems due to the selectors being sliced.

Objections?