Make dsnow non-optional.

[dabail10]

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PR checklist

• [X] Short (1 sentence) summary of your PR: Makes dsnow non-optional in icepack_step_therm1 call.
• [X] Developer(s): dabail10 (D. Bailey)
• [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. https://github.com/CICE-Consortium/Test-Results/wiki/cice_by_hash_forks#b109fa41023ac75bdaa3be820d5b076268b7670b
• 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?
  • [X] Yes
  • [ ] 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.)
  • [ ] Yes
  • [X] No, does the documentation need to be updated at a later time?
    • [ ] Yes
    • [X] No
• [X] Please document the changes in detail, including why the changes are made. This will become part of the PR commit log. I need an Icepack main to update the icepack here.

[apcraig]

We are going to defer this until after the BGC PR is merged, #968. I will convert this to draft. See also https://github.com/CICE-Consortium/Icepack/pull/497. Also, this PR has a change in Icepack which shouldn't be there. The icepack changes have to come in via an update to an Icepack main hash.