- 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