Schiller force model to be used instead of Guo on non-SRT

This is supposed to be equivalent to Guo, but confirmed to be working for non-SRT. Also includes two test cases:

• one that confirms momentum conservation for all available force models
• one that checks that the moments of the force look right, but only for Schiller and Simple force models

Fixes #11 (closed)

lbmpy_tests/test_force.py

@@ -80,7 +80,7 @@ def test_total_momentum(method, force_model, omega):
     assert np.allclose(total/np.prod(L)/F/t, 1)
 
 
-@pytest.mark.parametrize("stencil", ["D2Q9", "D3Q19", "D3Q27"])
+@pytest.mark.parametrize("stencil", ["D2Q9", "D3Q15", "D3Q19", "D3Q27"])
 def test_stress(stencil):
     """check Schiller's force term in mode space"""
     stencil = get_stencil(stencil)
@@ -119,7 +119,6 @@ def test_stress(stencil):
             for i in range(dim) for j in range(dim)}
     for force_moment, moment in zip(force_moments[dim+1:dim+1+num_stresses],
                                     method.moments[dim+1:dim+1+num_stresses]):
-        print(moment)
         ref = moment.subs(subs)
         diff = sp.simplify(ref - force_moment)
         assert diff == 0 or isinstance(diff, sp.Rational) # difference should be zero or a constant