- started with generalized free energy formulation

- added isotropic version

- removed warnings
- added flake8 as CI target

- test run again
- notebooks not yet

- 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

label -> target
ceIdx -> superscript

- merged finite difference functions into one function
- put derivative operators from lbmpy into pystencils

- 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

- added sphinx files for documentation generation
- collected kernel creation functions in new "cpu" and "cudagpu" modules