Add Hovmöller time vs. lat/lon plot to diagnostic `monitor/multi_datsets.py`

[lukruh]

See https://de.wikipedia.org/wiki/Hovm%C3%B6ller-Diagramm.

Aim: add new plot type hovmoeller_time_vs_lat_or_lon to monitor/multi_datasets.py.

Large parts of the necessary code can be copy-pasted from here: https://github.com/ESMValGroup/ESMValTool/blob/0d2b68d6614c667141207affd7834cc49d34b203/esmvaltool/diag_scripts/monitor/multi_datasets.py#L1418-L1494

Use /work/bd1179/esmvaltool_hackathon/recipes/recipe_hovmoeller_time_vs_lat_or_lon.yml to get started. You find more detailed instructions in this file.

TODO:

• [x] Discuss which variable/which regions/which observations we want to show.
• [x] adjust labels and order depending on lat or lon
• [x] documentation in diagnostic file
• [x] test meridional and zonal pp
• [x] single plot without reference
• [x] multi panel with reference and bias
• [x] consistent order in docs, methods, plot types etc..
• [x] try to enable/change all recipe options
• • [x] rasterize does not work
• • [x] remove show stats
• • [x] replace minor tick labels with minor ticks
• [x] update captions and provenance
• [x] discuss defaults and options with others working on hovmoellers
• [x] example recipes in monitor with some example options
• [x] example plots in docs doc/sphinx/source/recipes/recipe_monitor.rst add plot image to the bottom (compressed <100k)
• [x] flake, codespell, isort multi_datasets.py
• [x] add authors to recipe (and diag?)

[lukruh]

Example for Hovmoeller using iris: https://scitools-iris.readthedocs.io/en/stable/generated/gallery/meteorology/plot_hovmoller.html

[lukruh]

we should test _add_stats() again.