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

- new time loop caches all kernel functions with their argument dict
  -> inner loop just calls C functions with cached kwargs

- test run again
- notebooks not yet

- assignment collection
- sympyextensions

- getShearRelaxationRate is a free function now -> works also with cumulant methods
- better error message when calling kernels with wrong or too few parameters
- entropic & incompressible is not working by default due to pdf shift -> added NotImplemented exception
- new creation function for 'raw_mrt' where all relaxation rates can be independently specified
- enhanced entropic creation funtion, supports omega output field now