docs: def ratio_to_delta(self, isos_ss, ratio, oneover=False):
          '''
          Transforms an isotope ratio into a delta value
  
          Parameters
          ----------
          isos_ss: list or float
              list w/ isotopes, e.g., ['N-14','N-15'] OR the solar
              system ratio.
          ratio : float
              ratio of the isotopes to transform.
          oneover : boolean
              take the inverse of the ratio before transforming (never
              inverse of delta value!).  The default is False.
  
          Returns
          -------
          float
              delta value
  
          '''
          # define if isos_ss is the ratio or the isotopes
          if type(isos_ss) == float:
              ss_ratio = isos_ss
          elif type(isos_ss) == list:
              ss_ratio = self.inut.isoratio_init(isos_ss)
          else:
              print('Check input of isos_ss into ratio_to_delta routine')
              return None
  
          # check if one over is necessary or not
          if oneover:
              ratio = old_div(1,ratio)
  
          # calculate delta value
          delta = (old_div(ratio, ss_ratio) - 1.) * 1000.
  
          return delta