Prepare for Icepack1.5.0 release

[apcraig]

PR checklist

• Short (1 sentence) summary of your PR:
  Prepare for Icepack1.5.0 release
• Developer(s):
  apcraig
• 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.
  Tested a quick suite on derecho, will carry out significant pre release testing. Changed only one line of redundant code.
• 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 document the changes in detail, including why the changes are made. This will become part of the PR commit log.

Update version number to 1.5.0

Update interface documentation

Clean up redundant dsnown line in icepack_therm_vertical, see #506.