Fluctuating (Randomized, Thermalized) LBM

• get random number from external field, add kernel to test suite @winterhalter
• physical test case to check random number correlation (auto correlation function?) @kuron
• final version should compute random numbers in the kernel itself. 'mulhi' / 'mullo' functions are required - philox RNG - write a pystencils function similar to bitoperations for these operations - on x86 there are intrinsics for mulhi/mullo, alternatively one could also use AES instead of philox since there is also a intrinsic for that - on GPU there should also be an intrinsic - C fallback as one-liner - two versions required for drawing float & double random numbers - fast versions produce 2 or more RN per step, how to handle this?

Fixes walberla/walberla#80 (closed)