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

- removed warnings
- added flake8 as CI target

- test run again
- notebooks not yet

- single function to create kernel for specified target
- data type creation from string - reuse numpy functionality
- bugfixes in dot display

- added documentation and comparison table to force models
- fixed citation in sphinx documentation, bibliography has to be
  processed last -> renamed file, because processing is alphabetically

- 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

- square channel benchmark for evaluating spurious currents in Poseuille flow
- force model support in cumulant methods
- option to set initial velocity field for simple serial scenarios
- bugfix in simple force model

Michael Kuron authored Feb 13, 2017 and Martin Bauer committed Feb 13, 2017

This commit makes the Python code backwards compatible down to Python 2.7. Previously it would only run on Python 3.5 and up.

Problems fixed included:
- `time.perf_counter()` doesn't exist
- all classes need to be new-style
- `functools.lru_cache` doesn't exist
- only the last argument to a function call can be `*`-expanded
- the `nonlocal` keyword doesn't exist
- metaclasses are used with a different syntax
- `yield from` doesn't exist
- `tempdir.TemporaryDirectory` doesn't exist
- iterators need a `next()` method