Rename variables in collision kernels to avoid underscores

Motivation: Underscores in variable names are not allowed in Mathematica.

• populations fαβγ, example: fMMP
• the central moments mδεη, example: m210
• cumulants cmδεη, example: cm210

With α, β, γ ∈ {M, 0, P} and δ, ε, η ∈ {0, 1, 2}. The characters {M, 0, P} indicate the lattice directions {−1, 0, 1} and the characters {0, 1, 2} indicate the order of the moments or cumulants.

related issue on the naming of the directions: https://git.rz.tu-bs.de/irmb/VirtualFluids_dev/-/issues/107