code: def isochone_ratio(e, rd, r_hyp):
      
  
      if e == 0.:
          c_prime = 0.8
      elif e > 0.:
          c_prime = 1. / ((1. / 0.8) - ((r_hyp - rd) / e))
  
      return c_prime
docs: Get the isochone ratio as described in Spudich et al. (2013) PEER
      report, page 88.
  
      :param e:
          a float defining the E-path length, which is the distance from
          Pd(direction) point to hypocentre. In km.
      :param rd:
          float, distance from the site to the direct point.
      :param r_hyp:
          float, the hypocentre distance.
      :returns:
          c_prime, a float defining the isochone ratio