Caching sphinx directive figure outputs

The new ``:outname:`` property for the Sphinx plot directive can

be used to cache generated images. It is used like:

..code-block::rst

Without ``:outname:``, the figure generated above would normally be called,

e.g.:file:`docfile3-4-01.png` or something equally mysterious. With

``:outname:`` the figure generated will instead be named

:file:`stinkbug_plot-01.png` or even:file:`stinkbug_plot.png`. This makes it

easy to understand which output image is which and, more importantly, uniquely

keys output images to the code snippets that generated them.

Configuring the cache directory

The directory that images are cached to can be configured using the

``plot_cache_dir`` configuration value in the Sphinx configuration file.

If an image is already in ``plot_cache_dir`` when documentation is being

generated, this image is copied to the build directory thereby pre-empting

generation and reducing computation time in low-resource environments.