code: def _dens(self,R,z,phi=0.,t=0.):
          
          x,y,z= bovy_coords.cyl_to_rect(R,phi,z)
          if self._aligned:
              xp, yp, zp= x, y, z
          else:
              xyzp= numpy.dot(self._rot,numpy.array([x,y,z]))
              xp, yp, zp= xyzp[0], xyzp[1], xyzp[2]
          m= numpy.sqrt(xp**2.+yp**2./self._b2+zp**2./self._c2)
          return self._mdens(m)
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:
             2018-08-06 - Written - Bovy (UofT)