code: def _dens(self,R,z,phi=0.,t=0.):
          
          return self._mn3[0].dens(R,z,phi=phi,t=t)\
              +self._mn3[1].dens(R,z,phi=phi,t=t)\
              +self._mn3[2].dens(R,z,phi=phi,t=t)
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:
             2015-02-07 - Written - Bovy (IAS)