docs: def get_crs_variable(ds):
      '''
      Returns the name of the variable identified by a grid_mapping attribute
  
      :param netCDF4.Dataset ds: An open netCDF4 Dataset
      '''
      for var in ds.variables:
          grid_mapping = getattr(ds.variables[var], 'grid_mapping', '')
          if grid_mapping and grid_mapping in ds.variables:
              return grid_mapping
      return None