Use direct momentum in opt-in shifted blocked equilibrium

tests/unit/test_shifted_block_operators.py

@@ -19,6 +19,15 @@ def test_retained_density_and_shifted_planar_headers(tmp_path):
             if u==0:
                 np.testing.assert_allclose(a.sum(),delta,rtol=5e-8,atol=0)
                 if delta: assert np.any(a!=0)
+        elif kind=='J':
+            delta,jx,jy,ux,uy=np.array(numbers,dtype=np.float32).astype(float)
+            jd=cx*jx+cy*jy;ud=cx*ux+cy*uy
+            expected=w*(delta+3*jd+4.5*jd*ud-1.5*(jx*ux+jy*uy))
+            np.testing.assert_allclose(a,expected,rtol=3e-6,atol=2e-9)
+            actual=np.array([a.sum(),a@cx,a@cy,a@(cx*cx),a@(cx*cy),a@(cy*cy)])
+            target=np.array([delta,jx,jy,delta/3+jx*ux,jx*uy,delta/3+jy*uy])
+            np.testing.assert_allclose(actual,target,atol=4e-9,rtol=3e-6)
+            if jx==0 and delta: assert np.any(a!=0)
         else:
             mode=int(numbers[0]);rho,u,v=np.array(numbers[1:],dtype=np.float32).astype(float)
             b=np.fromstring(next(it),sep=' ',dtype=np.float32).astype(float)