Adds error check to IOStream for FileFreq options

[philipwjones]

When append mode is used, the FileFreq options must be supplied but an error check was missing to detect this case. User documentation was also revised to note the requirement and add some instructions for handling a special case.

Fixes #407

Checklist

• Documentation:
  • User's Guide has been updated
  • Documentation has been built locally and changes look as expected
• Linting
  • Pre-commit run locally
• Building
  • CMake build does not produce any new warnings from changes in this PR
• Testing
  • Add a comment to the PR titled Testing with the following:
    • Which machines CTest unit tests
      have been run on and indicate that are all passing.
    • The Polaris omega_pr test suite
      has passed, using the Polaris e3sm_submodules/Omega baseline
    • Document machine(s), compiler(s), and the build path(s) used for -p for both the baseline (Polaris e3sm_submodules/Omega) and the PR build
    • Indicate "All tests passed" or document failing tests
    • Document testing used to verify the changes including any tests that are added/modified/impacted.

[philipwjones]

CTests pass on PM cpu and gpu. Also tested error case when FileFreq options missing to verify abort and error message are triggered. Built revised docs and viewed them successfully.

[xylar]

Reviewed by inspection and @philipwjones's testing. Thanks @philipwjones! This looks like exactly what we need --both the error message and the updated docs.