There is unpysical flow inside of LB boundaries on the GPU, which exchanges no information with the physical LB fluid (possibly since Stefan rewrote the bounce back kernels). This commit fixes the problem, that particles close to a wall couple with this unphysical flow. In the coupling, the boundary fluid velocity is assumed to be 0. Thank you, Owen.
Merge remote-tracking branch 'origin/master' into lbgpu_particle_wall_fix