Sphinx gallery image scraper

Add a custom sphinx-gallery scraper for PNG files.

The scraper includes all PNG files generated by a particular block from an example.

We can use this to generate files using povray.

Checklist: x (done) or ~ (irrelevant)

• I am familiar with ASE's contribution guidelines.
• Doc strings in code changed in this MR are up to date.
• Unit tests have been added for new or changed code.
• Changes have been added in changelog.d using scriv.
• Issue is resolved via "closes #XXXX" if applicable.