use seed parameter to have results more reproducible

[haesleinhuepf]

This is an attempt to implement #86 but it seems to not work. https://github.com/haesleinhuepf/bia-bob/blob/e2f5094d7b43424761cde97903fe828d7cc7b299/demo/demo_seed.ipynb

As the seed parameter feature by OpenAI is beta, there is no hurry to make this work.

This is not meant to be merged right now.