code: def _dens(self,R,z,phi=0.,t=0.):
          
          r2= R**2.+z**2.
          rb= nu.sqrt(r2+self.b2)
          return (3.*(self.b+rb)*rb**2.-r2*(self.b+3.*rb))/\
              rb**3./(self.b+rb)**3./4./nu.pi
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:
             2013-09-08 - Written - Bovy (IAS)