code: def find_cached_dm(self):
          
          pmag_dir = find_pmag_dir.get_pmag_dir()
          if pmag_dir is None:
              pmag_dir = '.'
          model_file = os.path.join(pmag_dir, 'pmagpy',
                                    'data_model', 'data_model.json')
          # for py2app:
          if not os.path.isfile(model_file):
              model_file = os.path.join(pmag_dir, 'data_model',
                                        'data_model.json')
          if not os.path.isfile(model_file):
              model_file = os.path.join(os.path.split(os.path.dirname(__file__))[0],'pmagpy', 'data_model','data_model.json')
          if not os.path.isfile(model_file):
              model_file = os.path.join(os.path.split(os.path.dirname(__file__))[0], 'data_model','data_model.json')
          return model_file
docs: Find filename where cached data model json is stored.
  
          Returns
          ---------
          model_file : str
              data model json file location