see https://github.com/sympy/sympy/issues/16662

- replace_density_and_velocity simplification produced terms like
  0 * omega, because sympy's auto-eval is turned off
- sympys CSE routine can apparently only handle evaluated terms
- solution: evaluate multiplications with zero (i.e. replace them by 0)

- style changes marked by flake
- using newest kerncraft version

- don't use for production runs!

This restructuring allows for easier separation of modules into
separate repositories later. Also, now pip install with repo url can be
used.

The setup.py files have also been updated to correctly reference each
other. Module versions are not extracted from git state

- all branches are added up