code: def _evaluate(self,R,z,phi=0.,t=0.):
          
          r2= R**2.+z**2.
          r= nu.sqrt(r2)
          return (0.5*nu.log(1+r2/self._a2)\
                      +self._a/r*nu.arctan(r/self._a))/self._a
docs: NAME:
             _evaluate
          PURPOSE:
             evaluate the potential at R,z
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             Phi(R,z)
          HISTORY:
             2015-12-04 - Started - Bovy (UofT)