code: def _surfdens(self,R,z,phi=0.,t=0.):
          
          return 2.*integrate.quad(lambda x: self._dens(R,x,phi=phi,t=t),0,z)[0]
docs: NAME:
             _surfdens
          PURPOSE:
             evaluate the surface density for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the surface density
          HISTORY:
             2018-08-19 - Written - Bovy (UofT)