pymc3.py: make sure step is set within the model context manager
This patch fixes a bug in which the step method was not being set within the PyMC3 model context manager, so the code would not run if a step method was passed to the sampler.
This patch fixes a bug in which the step method was not being set within the PyMC3 model context manager, so the code would not run if a step method was passed to the sampler.
