cell stats

[bblay]

Adds cell statistics core functionality to cset with a new operator and plotter.

To run: From the temporary folder delme, run run_cell_statistics.py. It reads the cached retrieval data from RES' test_lfric config.

A comparison of RES and CSET output:

todo:

• HTML styling.
• Select the "all" time point by default, not just the first in the list (or make it first, even better).
• Axis labels, legend, etc.
• Why do the two plots above look similar/identical but the y axes don't match?
• The git commits were made with `--no-verify'.
• I didn't get as far as considering rose. I was just running from VDI.
• In the operator there's a variable called y_axis. It's not clear to me if the code this controls is still necessary.

Further thoughts:

• Should we make the operator return a CubeList? It currently returns a nested dict, five levels deep, because there's so many controls on the page. This led to the new pickle read and write operators, which helped speed up development.
• It would be nice if the read_cubes operator allowed multiple vars to be loaded so we don't have to load the files once per var. That would allow us to add a control for the variable, and would increase the depth of the nested dict.