WALBERLA: clean up langevin thermostat

[christophlohrmann]

The dry Langevin and the thermalised LB thermostat share the way they calculate friction and random noise from a friction parameter. Currently, the code is duplicated and the LB thermostat only has a part of the functionality (no particle-specific or anisotropic gamma).

The friction and noise calculation should moved to a function that both Langevin and LB thermostats can call.

[RudolfWeeber]

Christoph, would yo be willing to do this at the coding day?

[christophlohrmann]

I can give it a shot. Will we have the usual zoom-meeting for all participants?

[jngrad]

Most of the body of the function friction_thermo_langevin() from src/core/thermostats/langevin_inline.hpp needs to be extracted to a free function that can be re-used inside in the particle coupling code of the LB, such that anisotropic particles can get their noise matrix transformed from lab-frame to body-frame before coupling to the fluid. Currently, the LB coupling code only contains a copy of the isotropic component of the Langevin code in couple_particle() and lb_lbcoupling_calc_particle_lattice_ia() from src/core/grid_based_algorithms/lb_particle_coupling.cpp.

Adapt testsuite/python/thermostats_anisotropic.py to verify the coupling of anisotropic particles is correct.

[jngrad]

Closing since it was achieved as part of #4743.