Draft: Fix Schiller. Schiller vs Guo force test

Schiller needs shifted equilibrium velocity.

Test currently fails with deviations in the odd-order moments. No idea why. I think this test should pass.

lbmpy_tests/test_force.py

@@ -157,6 +157,31 @@ def test_modes(stencil, force_model):
         assert list(force_moments[dim+1:]) == [0] * (len(stencil)-(dim+1))
 
 
+@pytest.mark.parametrize("stencil", ["D2Q9", "D3Q15", "D3Q19", "D3Q27"])
+def test_schiller_vs_guo(stencil):
+    """compare Schiller's and Guo's force terms"""
+    stencil = get_stencil(stencil)
+    dim = len(stencil[0])
+    
+    omega = sp.Symbol("omega")
+    
+    F = [sp.Symbol(f"F_{i}") for i in range(dim)]
+    
+    method_s = create_lb_method(method="srt", stencil=stencil, relaxation_rate=omega, 
+                                compressible=True, force_model="schiller", force=F)
+    method_g = create_lb_method(method="srt", stencil=stencil, relaxation_rate=omega, 
+                                compressible=True, force_model="guo", force=F)
+    
+    force_moments_s = sp.simplify(method_s.moment_matrix * sp.Matrix(method_s.force_model(method_s)))
+    force_moments_g = sp.simplify(method_g.moment_matrix * sp.Matrix(method_g.force_model(method_g)))
+    diff = sp.simplify(force_moments_s - force_moments_g)
+    
+    print(method_s.moments)
+    print(diff)
+    
+    assert diff.transpose() == 0*diff.transpose()
+
+
 @pytest.mark.parametrize("force_model", force_models)
 def test_momentum_density_shift(force_model):
     target = 'cpu'