Squashed 'doc/_themes/sphinx_rtd_theme/' content from commit aa0d6f7

git-subtree-dir: doc/_themes/sphinx_rtd_theme
git-subtree-split: aa0d6f7abd0efda41565bdeb12a3ed303d5e584c
Merge commit '2127ee45ab9ff209d9114825beb3cc7dd50f12c5' as 'doc/_themes/sphinx_rtd_theme'
return root_seeds after following fluid

useful for daisy chaining follow_fluid calls?
fix 1d plotting from p2d
more openggcm reader improvements (parse run name into dataset info dict)
docs: switch to rtd theme?