Remove hc_jday and replace with "compute_days_between" for HYCOM forcing

[mhrib]

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PR checklist

• Short (1 sentence) summary of your PR:
  HYCOM forcing specific. Use "compute_days_between" from the newly introduced ice_calendar. And remove hc_jday, which is not used anymore
• Developer(s):
  @mhrib
• Suggest PR reviewers from list in the column to the right.
  @apcraig @eclare108213 @TillRasmussen
• Please copy the PR test results link or provide a summary of testing completed below.
  NOT done. However as DMI is the only one using this routine, I am confident, that my own checks is ok.
  ** update from automatic checks **
  19 measured results of 19 total results
  19 of 19 tests PASSED
  0 of 19 tests PENDING
  0 of 19 tests MISSING data
  0 of 19 tests FAILED
• 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 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 provide any additional information or relevant details below:
  HYCOM forcing specific only pull request:

1. Use "compute_days_between" from the newly introduced ice_calendar together with the 4 calendar public variables (from ice_in): year_init, month_init, day_init,sec_init
2. Remove hc_jday, which is not used anymore within the code.