code: def _surfdens(self,R,z,phi=0.,t=0.):
          
          if R > self.a: return 0.
          h= nu.sqrt(self.a2-R**2)
          if z < h: return 0.
          else: return 1./(2.*nu.pi*self.a*h)
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 density
          HISTORY:
             2018-08-04 - Written - Bovy (UofT)