parallelize histogram creation

[skuschel]

There are two ways to do this:

1. The outer for loops (for i in xrange(n)) should be parallelized within cython. This change is to be made in https://github.com/skuschel/postpic/blob/master/postpic/particles/_particlestogrid.pyx
2. The use of the cython function should be parallelized in python. This change is to be made in https://github.com/skuschel/postpic/blob/master/postpic/particles/_routines.py#L125

I suspect, that option 2 seems a little better to me, however benchmarking is needed. Benchmarks are already carried out as part of the tests.

this might help
http://www.dlr.de/sc/Portaldata/15/Resources/dokumente/PyHPC2013/submissions/pyhpc2013_submission_2.pdf

http://cs.nyu.edu/courses/spring15/CSCI-UA.0480-003/lecture13.pdf

[skuschel]

There is some proof-of-concept in the (outdated) cythonparallel branch. However it needs to be tests if this is actually increasing the performance.