Bugfix: Guo force model for non-SRT

An error was discovered as part of pycodegen/lbmpy!34 (merged), which made the Guo force model for non-SRT relaxation models and 2D lattices contribute incorrect values to the bulk stress.

Also, there was a sign error due to different definitions of the relaxation parameter between Ulf Schiller and Walberla. It was discovered in https://github.com/espressomd/espresso/pull/3885.