Set under/over indices for sea ice conc and thickness

[xylar]

This merge sets extend to both, rather than neither for difference plots of sea-ice concentration and thickness. The previous value of neither led to plots like this one:

In this PR have slightly updated the indices into the colormap and explicitly added indices for the unser/over (not necessary but there for clarity).

Checklist

• User's Guide has been updated
• Testing comment in the PR documents testing used to verify the changes

Fixes #1082

[cbegeman]

Addresses #1082

[xylar]

It's better to put the "Addresses ..." comment in the description of the PR because then it closes the issue when the PR gets merged. Normal comments unfortunately don't do that.

[xylar]

Testing

I ran a test from the test suite on this branch and I now see the expected over/under on thickness and concentration, e.g.:
https://web.lcrc.anl.gov/public/e3sm/diagnostic_output/ac.xasay-davis/analysis_testing/chrysalis/fix-sic-diff-over-under/main_py3.11/sea_ice/

I'm rerunning the rest of the suite now.

[xylar]

The main vs. ctrl run failed with out the fix in #1084. Trying again with it...

[xylar]

Okay, I have a working main vs. ctrl:
https://web.lcrc.anl.gov/public/e3sm/diagnostic_output/ac.xasay-davis/analysis_testing/chrysalis/fix-sic-diff-over-under/main_vs_ctrl/

[xylar]

@cbegeman and @darincomeau, could you give this a very quick review just by inspection and from whatever of my test suite results you want to look at? I don't want to waste a lot of your time but I also think it would be worth making sure this doesn't seem like it'll have unexpected side effects or something.

[cbegeman]

@xylar Looks good to me. Thanks!

[darincomeau]

@xylar looks good to me as well - thanks!

[xylar]

Great, thanks!