code: def get_phi_ss(imt, mag, params):
      
      C = params[imt]
      if mag <= 5.0:
          phi = C["a"]
      elif mag > 6.5:
          phi = C["b"]
      else:
          phi = C["a"] + (mag - 5.0) * ((C["b"] - C["a"]) / 1.5)
      return phi
docs: Returns the single station phi (or it's variance) for a given magnitude
      and intensity measure type according to equation 5.14 of Al Atik (2015)