Fix: momentum density calculation

Fixes the momentum density calculation for compressible methods with force model. Before the macroscopic velocity shift was used, now we introduce the macroscopic momentum density shift to avoid dividing by the density twice in the force correction step.

Solves issue #14 (closed).

lbmpy_tests/test_force.py

@@ -177,7 +177,7 @@ def test_momentum_density_shift(force_model):
     src = dh.add_array('src', values_per_cell=len(stencil))
     dh.fill('src', 0.0, ghost_layers=True)
 
-    method = create_lb_method(method="srt", compressible=True, force_model='guo', force=[1, 2])
+    method = create_lb_method(method="srt", compressible=True, force_model=force_model, force=[1, 2])
 
     momentum_density_symbols = sp.symbols("md_:2")
     cqc = method.conserved_quantity_computation