docs: def set_ref(self, ref_key, ref_id):
          """
              Using a ref key and ref id set the
              reference to the appropriate resource type.
          """
          if ref_key == 'NETWORK':
              self.network_id = ref_id
          elif ref_key == 'NODE':
              self.node_id = ref_id
          elif ref_key == 'LINK':
              self.link_id = ref_id
          elif ref_key == 'GROUP':
              self.group_id = ref_id
          elif ref_key == 'SCENARIO':
              self.scenario_id = ref_id
          elif ref_key == 'PROJECT':
              self.project_id = ref_id
  
          else:
              raise HydraError("Ref Key %s not recognised."%ref_key)