Draft: [Fix] Read velocity from field in UBB

In the UBB boundary condition, the velocity is taken from neighbouring points when a field is provided. However, this also results in prescribing a different velocity for PDFs of the same cell which can not be intentional. With this MR the velocity is taken from the source cell instead

src/lbmpy/boundaries/boundaryconditions.py

@@ -800,10 +800,7 @@ class UBB(LbBoundary):
 
         neighbor_offset = NeighbourOffsetArrays.neighbour_offset(dir_symbol, lb_method.stencil)
 
-        velocity = tuple(v_i
-                         if isinstance(v_i, Field.Access) and not vel_from_idx_field
-                         else v_i
-                         for v_i in vel)
+        velocity = tuple(v_i for v_i in vel)
 
         if self._adaptVelocityToForce:
             cqc = lb_method.conserved_quantity_computation