Introduce new diag_mode option to MARBL_generate_diagnostics_file.py; when
diag_mode = 'none', output diagnostic file should be 'never_{op}' for all
variables. When diag_mode = 'minimal', only variables with diag_mode =
'minimal' in diagnostics_latest.json have non-never output frequencies. When
diag_mode is 'full', variables with diag_mode = 'minimal' or 'full' will have
non-never output frequencies.

Call MARBL_generate_diagnostics_file.py with a few different --diag-mode
options (and write output to different files)

Previous commit let us call MARBL_generate_diagnostics_file.py from command
line, but caused problems when importing generate_diagnostics_file() into a
python script. This clean-up allows both use-cases.

Also, I renamed diag_mode_opts() -> valid_diag_modes() because that seems like
a more descriptive name.

When generating the MARBL diagnostics file with diag_mode=minimal, some
variables were being listed as "never_average, never_average" (and others were
"medium_average, never_average"); we only need to add never_average to the
freq_op list if that list is empty.

1) Cleaned up docstring for valid_diag_modes
2) Dropped valid_diag_modes from __init__.py
3) Moved imports to top of MARBL_generate_diagnostics_file.py
   -- some are inside if __name__ == "__main__" block
   -- I snuck in a sys.path.append() in the middle of the imports to get
      MARBL_tools into the path