Windows 10 multicore workaround

[rwalke]

Hi,
Thanks for doing this great job!
I am trying to run this computation on a MS Window 10 system.
If I have more than one core my Jupyter-Notebook-Kernel crashes.
But, it helps to restrict the number of cores to one, e.g.
trace = pm.sample(model=model, init='advi', cores=1)
Best regards,
Rainer, MPIDR