Add ensemble panel plots to support RRFSE.

[christinaholtNOAA]

This reorganizes some of the code to slim-down create_graphics.py to organize data and setup parallelism for the most part. Utilities and graphics-building have been moved out to the adb_graphics package.

The code now supports plotting 12-panel plots to support 10-member forecast ensembles for RRFSE. It will also allow composite reflectivity observations to be plotted on the bottom left panel for those figures.

@cshartsough contributed the design aspect to this work, gathering feedback from the interested AVID ensemble folks and providing the prototypes for how the figures would be built.

Because this is a big structural change, I'll add some comments in-line after I open the PR.

The code has currently been tested for the ensemble case for the 7 major domains, and the same list of fields plotted as a 9-member ensemble in 2021. I will include examples below.

The tests to ensure single figures are still working are pending.

[christinaholtNOAA]

Examples!

Note: The ensemble data and observation data are from separate valid times and are only meant to show design aspects!

[christinaholtNOAA]

@Brian-Jamison The observations are the composite reflectivity observations. They come to us in grib2 format, so that was a mostly straightforward addition. It requires reading in an additional file, but nothing too major.