• Use add_gridspec to construct the gridspec and subgridspec to
  construct the subgridspec, saving an import.
• Use nested loops rather than itertools.product, which is nice but
  not really the point of the tutorial.
• Index gridspecs with two indices rather than a single flat index,
  again matching "standard" use.
• Set spine visiibility to the right value directly, rather than first
  toggling them all off and then a few on again.

PR Summary

PR Checklist

• Has Pytest style unit tests
• Code isFlake 8 compliant
• New features are documented, with examples if plot related
• Documentation is sphinx and numpydoc compliant
• Added an entry to doc/users/next_whats_new/ if major new feature (follow instructions in README.rst there)
• Documented in doc/api/api_changes.rst if API changed in a backward-incompatible way