Can't output _only_ `gyre_overturn_diagnose` diagnostics in a file

[dougiesquire]

It seems that the diagnostics from the gyre_overturn_diagnose routine cannot be the only diagnostic(s) in an output file. Doing so results in those diagnostics silently being incorrect. Adding any additional (non-gyre_overturn_diagnose) diagnostic to the file seems to fix the problem.

See this issue for a real example.