[IMP-dev] atom module

[Daniel Russel <drussel@gmail.com>]

Since we have various atomic functionality and are getting more, we  
should probably start the atom module. Initially it will just have
AtomDecorator
ResidueDecorator
MolecularHierarchyDecorator

I would be inclined to put a wrapper for the modeller pdb reader there  
too, so that we don't have modeller dependencies scattered throughout  
the example files.