Shifted the selection object out of pipe_control.mol_res_spin and into tne new lib.selection module.
The dependence on the MoleculeContainer, ResidueContainer and SpinContainer objects have been
removed, as this is part of the relax data store. Therefore all of the private methods
(__contains__, __contains_mol_res_spin_containers, and __contains_spin_id) have been deleted. The
contains_*() will need to be used instead.