Make sure all relations (po_2, bl_1, etc.) are part of the returned dict of DFs

Basically make sure that all the required relations: 'po_2', 'u_1', 'bl_2', 'pp_2', 'rel_3', 'dr_2', 'eq_2' are part of the dict of DFs that are returned, otherwise for example the visualization func might error out claiming it couldn't find a particular matrix. So before returning, if one of these is not present, then add an empty DF for that relation.