Integrate force calculation in wall boundaries

This MR provides the possibility to calculate the force that a fluid creates on a resting wall. The calculation is integrated in the kernel

src/lbmpy/boundaries/boundaryconditions.py

@@ -126,7 +126,7 @@ class NoSlip(LbBoundary):
     def __call__(self, f_out, f_in, dir_symbol, inv_dir, lb_method, index_field, force_vector):
         if self.calculate_force_on_boundary:
             force = sp.Symbol("f")
-            subexpressions = [Assignment(force, f_in(inv_dir[dir_symbol]) + f_out(dir_symbol))]
+            subexpressions = [Assignment(force, sp.Float(2.0) * f_out(dir_symbol))]
             offset = NeighbourOffsetArrays.neighbour_offset(dir_symbol, lb_method.stencil)
             for i in range(lb_method.stencil.D):
                 subexpressions.append(Assignment(force_vector[0](f'F_{i}'), force * offset[i]))
@@ -207,7 +207,7 @@ class NoSlipLinearBouzidi(LbBoundary):
 
         if self.calculate_force_on_boundary:
             force = sp.Symbol("f")
-            subexpressions.append(Assignment(force, f_in(inv_dir[dir_symbol]) + rhs))
+            subexpressions.append(Assignment(force, f_xf + rhs))
             offset = NeighbourOffsetArrays.neighbour_offset(dir_symbol, lb_method.stencil)
             for i in range(lb_method.stencil.D):
                 subexpressions.append(Assignment(force_vector[0](f'F_{i}'), force * offset[i]))
@@ -344,7 +344,7 @@ class QuadraticBounceBack(LbBoundary):
 
         if self.calculate_force_on_boundary:
             force = sp.Symbol("f")
-            subexpressions.append(Assignment(force, f_in(inv_dir[dir_symbol]) + result))
+            subexpressions.append(Assignment(force, f_xf + result))
             offset = NeighbourOffsetArrays.neighbour_offset(dir_symbol, lb_method.stencil)
             for i in range(lb_method.stencil.D):
                 subexpressions.append(Assignment(force_vector[0](f'F_{i}'), force * offset[i]))