Update floe size distribution in Icepack

[eclare108213]

Additional Icepack commits from @lettie-roach's ifsd2 branch

PR checklist

• Short (1 sentence) summary of your PR:
  Updating FSD in Icepack, cherry-picked commits from ifsd2 branch for a clean merge.
• Developer(s):
  @lettie-roach @eclare108213
• 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.
  base_suite without regression:
  101 of 101 tests PASSED
  0 of 101 tests FAILED
  0 of 101 tests PENDING
  Test results
• 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:

These changes include some refactoring of FSD calculations to reduce the number of flops, which will cause the FSD cases to be non-BFB. The default configurations should still be BFB. As long as @lettie-roach is happy with the results, I do not think we need to conduct QC testing for these FSD changes.

@apcraig: I'd appreciate it if you'd run a full suite of standard tests with regression.
@lettie-roach: If you'll also test and approve these changes to icepack, we will merge it. Is the comment on line 817 of icepack_fsd.F90 still correct? (the 0.5 factor)

The next step is to replace the current icepack submodule in cice with the new icepack (@apcraig), and then I will update the cice FSD PR, which we can all test as a final sanity check.

[lettie-roach]

I have done a year-long run with 12 floe size categories and wave-ice interactions, and am happy with the results.

[eclare108213]

Once travis finishes, this PR will be ready for a final suite of regression tests and merging.

[apcraig]

OK, I'll start my testing.

[apcraig]

Testing looks good. It's bit-for-bit with a version from a couple weeks ago and passes full test suites on izumi and gordon. See https://github.com/CICE-Consortium/Test-Results/wiki/icepack_by_hash_forks hash 6167eba. I will merge this now.