Add warning for FSD without waves, clarify documentation

[lettie-roach]

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PR checklist

• Short (1 sentence) summary of your PR:
  Add warning for FSD without waves, clarify documentation
• Developer(s):
  Lettie Roach
• Suggest PR reviewers from list in the column to the right.
  @eclare108213 @apcraig
• Please copy the PR test results link or provide a summary of testing completed below.
  None
• 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:

(@eclare108213 edited to to remove spaces from the checkboxes)

[eclare108213]

This can be merged as soon as we make the new wave forcing file available, along with the CICE changes in CICE-Consortium/CICE#775.

[apcraig]

This all looks pretty straight-forward, not sure where a new file comes in. I think this can be merged as is unless there are other things to do.

[lettie-roach]

Is there already a wave spectrum file for a whole year available? If so, then no extra file is needed. Cheyenne is down at the moment so I can't check what's in the ocn_data_dir

[apcraig]

The file we're using now for testing is ww3.20100101_efreq_remapgx3.nc. That file seems to have 6 hourly data for 1 day.

	time = UNLIMITED ; // (4 currently)
		time:units = "days since 1990-01-01 00:00:00" ;
 time = 7305, 7305.25, 7305.5, 7305.75 ;