Revert !146: remove Su viscoelasticity
This MR reverts !146 (merged). Due to changes in the Guo force model for TRT the reference solution for the Su model is not correct anymore.
Furthermore, the force models in lbmpy have been adapted to only work with symbols internally. This needed some adaption in waLBerla.
Fixes #158 (closed)
Edited by Michael Kuron