Add an IMP.atom.get_leaves(hier) which is aware of representations

[cgreenberg]

Currently the only way to get ALL leaves of all representations is a bit clunky, you have to use selection:

selB = IMP.atom.Selection(hsel,resolution=IMP.atom.ALL_RESOLUTIONS,
                                  representation_type=IMP.atom.BALLS)
selD = IMP.atom.Selection(hsel,resolution=IMP.atom.ALL_RESOLUTIONS,
                                  representation_type=IMP.atom.DENSITIES)
return list(set(selB.get_selected_particles() + selD.get_selected_particles()))

Would be nice to have a quick function to do this.