Make dsnow non-optional.

[dabail10]

For detailed information about submitting Pull Requests (PRs) to the CICE-Consortium,
please refer to: https://github.com/CICE-Consortium/About-Us/wiki/Resource-Index#information-for-developers

PR checklist

• Short (1 sentence) summary of your PR:
  Makes dsnow non-optional in icepack_step_therm1 call.
• Developer(s):
  dabail10 (D. Bailey)
• Suggest PR reviewers from list in the column to the right.
• 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?
  • bit for bit
  • different at roundoff level
  • more substantial
• Does this PR create or have dependencies on Icepack or any other models?
  • 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
  • No
• Does this PR add any new test cases?
  • Yes
  • 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
  • No, does the documentation need to be updated at a later time?
    • Yes
    • No
• 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 CICE-Consortium/Icepack#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.