Do not run Python demos each time docs are generated

[garth-wells]

To avoid storing image files in the repository, the pre-processing stage of the documentation generations runs the Python demos that generate images for inclusion in the docs. What is tedious is that the image-generating demos are run every time the the docs are generated, and not only the first time.

Possible solution include:

1. Only run the demos when the image files are not available.
2. Add a command line option to run the demos (probably a better option).