code: def _dens(self,R,z,phi=0.,t=0.):
          
          r2= R**2+z**2
          if r2 != self.a2:
              return 0.
          else: # pragma: no cover
              return nu.infty
docs: NAME:
             _dens
          PURPOSE:
             evaluate the 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)