Resolve "Enable animation in facet_grid plot through 'frames' key"

This MR adds the frames specifier to the facet_grid plot function, thus allowing a new dimension along which to represent data.

Details

This required quite some restructuring of the existing function, and a lot of @herdeanu 's time and pain to understand the various scenarios in which new Figure objects are created, re-used, whatever – or in which return values suddenly change types... very obscure.

Highlighted changes:

• facet_grid now accepts frames argument
• Inside, the plot_frame helper generalises a single plot, given some data
• An update function is added, that uses the d.groupby(frames) operation and the plot_frame helper to yield new frames.
• To prevent unused figure objects aggregating, a figure_leak_prevention context manager is added and used

Anything to double-check?

• @blsqr's changes to @herdeanu's code ok?
• How to organise proper code attribution?
• Tests complete? Add more test cases?
• Run the FULL_TEST locally and check the output, perhaps

Can this MR be accepted?

• Implementation ready
• Tests added or adjusted
• Documentation extended or updated

• Code quality
  • Changes follow coding guidelines
  • Checked code coverage on new and adjusted code

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped (not necessary; but this feature should be the last in v0.12.3 ‼ )
  • Reviewed & approved by @blsqr
  • Reviewed & approved by @herdeanu

Related issues

Closes #142 (closed)