Output streams in append mode require `FileFreq`

[xylar]

As discussed in E3SM-Project/polaris#572 (review), we're seeing an issue where an output stream overwrites a file even with:

    History:
      UsePointerFile: false
      Filename: output.nc
...
      IfExists: append

Adding:

      FileFreq: 9999
      FileFreqUnits: years

seems to fix the issue but this is not an intuitive requirement, since we plan to use a single file for output.