code: def _dens(self,R,z,phi=0.,t=0.):
          
          return 1./(1.+(R**2.+z**2.)/self._a2)/4./nu.pi/self._a3
docs: NAME:
             _dens
          PURPOSE:
             evaluate the density for this potential
          INPUT:
             R - Galactocentric cylindrical radius
             z - vertical height
             phi - azimuth
             t - time
          OUTPUT:
             the density
          HISTORY:
             2015-12-04 - Started - Bovy (UofT)