Walberla: Support OpenMP

[RudolfWeeber]

For systems with a low number of particles <1000, it may not be worth it to run MPI parallelizaiton on the particle side. However, the LB can run in parallel using OpenMP.

• Enable this in the code generation of the kernels
• Add a omp_set_num_threads(1) in on_program_start() or the like
• Add a function in the espressomd python pakcage (e.g. via _init.py) to change the number of threads