Update U10 to be resolved wind; add variable for U10+gusts

[megandevlan]

Description of changes

This PR revises the definition of U10 to be only the large-scale resolved winds, as was the case before gusts were added to CMEPS/CAM. The merged U10 (large-scale + gusts) is now output as a separate variable.

Specific notes

Note that duu10n is still reflective of the merged U10, as this is the wind speed used to compute actual surface fluxes. But the U10 that's output is back to being un-merged.

Contributors other than yourself, if any:
Credit to @quantheory for pointing out the issue and developing a fix for E3SM here E3SM-Project/E3SM@5813d41

CMEPS Issues Fixed (include github issue #):
#438

Are changes expected to change answers? (specify if bfb, different at roundoff, more substantial)
The change in U10 could shift some things.

Any User Interface Changes (namelist or namelist defaults changes)?
Not on the CMEPS side.

Testing performed

Please describe the tests along with the target model and machine(s)
If possible, please also added hashes that were used in the testing
Tested on Derecho in a global configuration, using tag cesm2_3_17b.

[megandevlan]

The second piece of this PR ("Fix calculation of gustiness") is the critical piece at this point. The modification follows the new formulation we're using to add gustiness to U10, wherein the gust component is moved inside the square-root calculation of vmag:
vmag = max(seq_flux_atmocn_minwind, sqrt( (ubot(n)-us(n))**2 + (vbot(n)-vs(n))**2 + (1.0_R8*ugust(min(rainc(n),6.94444e-4_r8))**2)) )

[cacraigucar]

@jedwards4b - This is a fix that folks want in before @cecilehannay starts up her runs (and before cesm2_3_beta17 is made)

[jedwards4b]

LGTM

[cacraigucar]

@jedwards4b - Is there a tag for these changes (so I can use them in CAM)?

[jedwards4b]

cmeps0.14.55