Show output while running notebooks

This uses our newly created module Princess to run the notebook before calling nbconvert to convert it to RST. This does two things

• The output is visible in the Slurm .out files while the notebook is still running - helping to understand code which hangs or takes a long time to run.
• It ensures that the code in the notebook runs in the same Python environment that pycalibration is installed in. At the moment, this is normally the case, but it depends on the kernelspecs Jupyter finds.

Doing this needed a new version of jupyter_client.