- update waLBerla package in order to make use of this function!

- small (length < 5) arrays with shape and stride information had to be
  memcpy'd to the GPU before every kernel call
- instead of passing the information as arrays, the single elements are
  passed
- leads to more function arguments, but simplifies GPU kernel calls

-> changes in all backends required

- fix for field collection and reduction operations
- support for aligned fields in parallel data handling

- new time loop caches all kernel functions with their argument dict
  -> inner loop just calls C functions with cached kwargs

- support aligned load/stores
- nontemporal stores
- aligned memory allocation for arrays and temporary buffers

- test run again
- notebooks not yet

-> one place to switch between cpu and gpu

- step class for LB phasefield generic enough to work with 3-phase and
  N-phase models
- cahn hilliard can either be solved by LBM or by finite differences
- 3 phase model can be solved with rho phase or without

- 3 phase model works now with step sytem

- plotting
- calculation of force on boundary
- vtk outpu

- Step working for serial CPU scenarios

- scaling interface width eta instead of surface tensions tau to correct
  interface profile & surface tensions

because boundary handling does its own flag management