Use a local RNG for reproduceable results

[Permafacture]

This is the recommended practice for using numpy random numbers. Setting an RNG within the function means the results will always be reproduce-able regardless of what other functions in the main program are also consuming random numbers

[jaroslavknotek]

@pvigier Is it possible to merge this PR? There are no breaking changes involved therefore release should be smooth.

[mcejp]

For what it's worth, I added this in my fork, which is also published on PyPI :)