This pull request allows the velocity bounce-back boundaries to (optionally) store the force that acts on them. The calculation is the same as in https://i10git.cs.fau.de/walberla/walberla/blob/master/src/pe_coupling/momentum_exchange_method/boundary/SimpleBB.h and the test case is ported from https://github.com/espressomd/espresso/blob/python/testsuite/python/lb_stokes_sphere.py.
This features was requested by @abaurreavelasco and by the Espresso developers.