Pyndl

- pyndl should now be installable on MacOS without openmp support
- splits `ndl_parallel.pyx` into several files to separate the openmp code from the rest
- some maintenance improvements

* fixes math.ceil with explicit cast to double before ceiling

* makes error codes in ndl_parallel more human readable

* fixes error code -166... (issue 85 on github)
* the (now fixed) error lead to pyndl not running under specific circumstances
* general code improvements

- fixes encoding error in long description of setup.py
- a lot of small code improvements
- improvements in the maintenance code

* fixes integer overflow in uni-dimensional index of weights in ``ndl.ndl`` that results in wrong learning in weight matrices with more than 4294967295 cells
* please relearn any weights where number of cues times numbers of outcomes exceeds 4294967295 (sorry for this bug)
* thanks to Yuying for finding it

- Moves code to read and write events into ``pyndl.io`` module. This changes the existing API! Deprecation warnings are added accordingly.
- improves documentation