[IMP-dev] Moving atom and residue types

I moved the atom and residue predfined types from being static members of the respective decorator (i.e IMP::core::AtomDecorator::AT_C) to being members of the IMP::core namespace (IMP::core::AT_C) so that swig could manage to export them to python. This shouldn't affect too much code as they were not previously usable from python, but just be aware.