code: def _evaluate(self,R,z,phi=0.,t=0.):
          
          if self.isNonAxi:
              return 1./2.*nu.log(R**2.*(1.-self._1m1overb2*nu.sin(phi)**2.)
                                  +(z/self._q)**2.+self._core2)
          else:
              return 1./2.*nu.log(R**2.+(z/self._q)**2.+self._core2)
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:
             2010-04-02 - Started - Bovy (NYU)
             2010-04-30 - Adapted for R,z - Bovy (NYU)