Feature/multiprocessing

For faster matrix generation
This should also become `range`. This in turn might have a performance impact on Python2. Not sure if we should take that into consideration or not.

Until now, there were no reasons for a compatibility layer like `six`. And I don't quite feel happy to use it for just one single call to `xrange`. If *really* necessary, we could use `sys.hexversion` (or something similar) at the module level to alias `xrange` to `range` if necessary:

    if sys.hexversion < 0x030000F0:
        range = xrange

... or something along those lines (I haven't tested this). This would only have a minimal impact at import-time (and as such negligible).