Update the calculation of uvel and vvel in evp dynamics

[apcraig]

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PR checklist

• [X] Short (1 sentence) summary of your PR: Update the calculation of uvel and vvel in evp to reduce flops, bit-for-bit.
• [X] Developer(s): apcraig, jflemiuex, eclare
• [X] Suggest PR reviewers from list in the column to the right.
• [X] Please copy the PR test results link or provide a summary of testing completed below. Full test suites run on derecho with intel, gnu, cray. All testing passes as expected, https://github.com/CICE-Consortium/Test-Results/wiki/cice_by_hash_forks#c5b390ec0507633c319e6c7d1cb2a6850924dc59
• How much do the PR code changes differ from the unmodified code?
  • [X] bit for bit
  • [ ] different at roundoff level
  • [ ] more substantial
• Does this PR create or have dependencies on Icepack or any other models?
  • [ ] Yes
  • [X] No
• Does this PR update the Icepack submodule? If so, the Icepack submodule must point to a hash on Icepack's main branch.
  • [ ] Yes
  • [X] No
• Does this PR add any new test cases?
  • [ ] Yes
  • [X] No
• Is the documentation being updated? ("Documentation" includes information on the wiki or in the .rst files from doc/source/, which are used to create the online technical docs at https://readthedocs.org/projects/cice-consortium-cice/. A test build of the technical docs will be performed as part of the PR testing.)
  • [X] Yes
  • [ ] No, does the documentation need to be updated at a later time?
    • [ ] Yes
    • [ ] No
• [X] Please document the changes in detail, including why the changes are made. This will become part of the PR commit log.

Update the calculation of uvel and vvel in subroutine evp in file ice_dyn_evp.F90. Do an unmasked grid_average_X2Y when averaging from uvelE to uvel and from vvelN to vvel instead of a masked average. The masking is handled separately. This change is bit-for-bit and saves a few flops. Closes #952.

Update the documentation to add the hemispheric sign dependence in the water stress terms in the dynamics equations. Closes #951.

[apcraig]

I'm doing a full test suite to make sure answers haven't changed.

@JFLemieux73 @eclare108213 Please review code changes and documentation, let me know if I've done something wrong.

[apcraig]

Testing looks good, https://github.com/CICE-Consortium/Test-Results/wiki/cice_by_hash_forks#c5b390ec0507633c319e6c7d1cb2a6850924dc59.