code: def _surfdens(self,R,z,phi=0.,t=0.):
          
          return self._a*(R**2+self._a**2)**-1.5/2./nu.pi
docs: NAME:
             _surfdens
          PURPOSE:
             evaluate the surface density
          INPUT:
             R - Cylindrical Galactocentric radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             Sigma (R,z)
          HISTORY:
             2018-08-19 - Written - Bovy (UofT)