WIP: Fluctuating MRT

This merge request copies @winterhalter's effort from the old lbmpy repository (https://i10git.cs.fau.de/software/pystencils/merge_requests/37). The main things that are still missing are tests and generalization to non-D3Q19.

I guess this still needs https://i10git.cs.fau.de/software/pystencils/merge_requests/26 to be brought forward from the old repository as it doesn't generate any RNG calls.

Fixes #2 (closed). Related to walberla/walberla#80 (closed).