GitLab

- allow AST nodes in assignment collection to e.g. put RNG nodes in
  LB method

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

- sweep updates pdf field in place, but does not load all the values
  before overwriting them
-> added a new transformation on AssignmentCollection that loads all
   read values first

- removed warnings
- added flake8 as CI target

- assignment collection
- sympyextensions

- Previously sympy.Eq was used to represent assignments. However Eq
  represents equality not assignment. This means that sometimes sympy
  "simplified" an equation like a = a  to True,
-> replaced sp.Eq by pystencils.Assignment everywhere
- renamed EquationCollection to AssignmentCollection