- removed warnings
- added flake8 as CI target

- test run again
- notebooks not yet

- notebooks are considered in coverage analysis now
   -> custom notebook runner plugin adapted
- added images for documentation

-> 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