code: def adjust_obscov_resfile(self, resfile=None):
          
          self.pst.adjust_weights_resfile(resfile)
          self.__obscov.from_observation_data(self.pst)
docs: reset the elements of obscov by scaling the implied weights
          based on the phi components in res_file so that the total phi
          is equal to the number of non-zero weights.
  
          Parameters
          ----------
          resfile : str
              residual file to use.  If None, residual file with case name is
              sought. default is None
  
          Note
          ----
          calls pyemu.Pst.adjust_weights_resfile()