A few fixes to climatology maps of velocity

[xylar]

The colormaps in difference plots are now being set.

Several docstrings and comments copied from SOSE have been fixed.

The name of the gallery group has been fixed for main vs. ctrl runs.

Checklist

• Testing comment in the PR documents testing used to verify the changes

[xylar]

Testing

I ran the test suite on Chrysalis. Results are here:
https://web.lcrc.anl.gov/public/e3sm/diagnostic_output/ac.xasay-davis/analysis_testing/chrysalis/fix-velocity-difference-colormap/

The mesh_rename test didn't work but for reasons unrelated to this PR that I will address elsewhere. Update: this appears to just have been related to a bad node.

The main vs. ctrl run produced expected results:

Before, this plot looked like this:

The gallery group is now Global Velocity, (I removed Control: ctrl after the testing, so it is still there in the web pages above) whereas it was Global Control: ctrl before.

[xylar]

@cbegeman, I ran across a few issues here while running tests for E3SM-Unified. I'd appreciate a quick review by inspection when you have a few minutes.

[cbegeman]

@xylar Thanks for fixing this! I noticed those issues but hadn't gotten around to addressing them.

[xylar]

@cbegeman, thanks for having a look!