code: def _dens(self,R,z,phi=0.,t=0.):
          
          r= nu.sqrt(R**2.+z**2.)
          return 1./r**self.alpha*nu.exp(-(r/self.rc)**2.)
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:
             2013-06-28 - Written - Bovy (IAS)