Reduce number of calls to calc_ffrac

[TillRasmussen]

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PR checklist

• Short (1 sentence) summary of your PR:
  step_a calls calc_ffrac 6 times. These are two by two very similar. In order to half the number of calls and remove an if statement the function has been slightly refactored. step_a does not consume much time, thus this cannot be seen in the total timing.
• Developer(s):
  tar
• Suggest PR reviewers from list in the column to the right.
  @eclare108213 @apcraig @JFLemieux73
• Please copy the PR test results link or provide a summary of testing completed below.
• Testing has been forced to use eap in all cases
  #totl = 1214 total
  #chkd = 1214 checked
  #pass = 1198
  #pend = 14
  #miss = 2
  #fail = 0
  #failbuild = 0
  #failrun = 0
  #failtest = 0
  #failcomp = 0
  #failbfbc = 0
  #failgen = 0
  ~
• How much do the PR code changes differ from the unmodified code?
  • [ x] bit for bit
  • different at roundoff level
  • more substantial
• Does this PR create or have dependencies on Icepack or any other models?
  • Yes
  • [ x] No
• Does this PR add any new test cases?
  • Yes
  • [ x] 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
  • [x ] No, does the documentation need to be updated at a later time?
    • Yes
    • [x ] No
• Please provide any additional information or relevant details below:

[apcraig]

Just as a check, I ran full test suites with 3 compilers on cheyenne and everything is bit-for-bit. I'll merge now.