Fix a bug with the swredist code in ktherm=2.

[dabail10]

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PR checklist

• Short (1 sentence) summary of your PR:
  This fixes a bug in the ktherm=2 code where dt_rhoi_hlyr was not computed. Raised in Issue bug in icepack_therm_mushy CICE#503. I have also added a test for this.
• 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/icepack_by_mach_forks#izumi
• 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 CICE or any other models?
  • 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/.)
  • Yes
  • No, does the documentation need to be updated at a later time?
    • Yes
    • No
• Please provide any additional information or relevant details below:
  This is bfb in all the existing test cases. I have added a new test case for ktherm=2 and swredist = .true. CICE will need to update this Icepack tag.

[eclare108213]

wasn't dg_adding_diagnostics.rst already added in another PR?

[dabail10]

That should not have been there. Sorry about that.