bug: perfect_model_obs namelist options cause true_state.nc to be written when you don't want any state output.

🐛 🌵

Describe the bug

1. Run perfect_model_obs
   with the following namelist options,

 write_output_state_to_file = .false.,
 single_file_out            = .true.

2. You've set write_output_state_to_file = .false., because you don't want to write out state files because you're running perfect_model_obs to get obs_seq.out. So you think you don't have to worry what single_file_out is set to.

3. What actually happened?

   • true_state.nc gets written - which performance wise you are not interested in writing out the state you just read in and did not change.
   • and in the case of MOM6 you get:

PE 0: p_m_o trace: Before setup for next group of observations
PE 0: p_m_o trace: Number of observations to be evaluated 49953
p_m_o trace: Time of first observation in window day=149748, sec=43275
p_m_o trace: Time of last observation in window day=149749, sec=43200
PE 0: p_m_o trace: After setup for next group of observations
PE 0: p_m_o trace: Before updating true state
PE 0: file_to_text input.nml file shape is 1133 by 243 truncating to 1133
by 129
ERROR FROM:
source : netcdf_utilities_mod.f90
routine: nc_append_time
message: put_var time: NetCDF: Not a valid data type or _FillValue type mismat
ch
message: ...

Which model(s) are you working with?

MOM6, but I think true_state.nc would get written for any model.

Version of DART

Which version of DART are you using?
v11.11.1-1-gca200df36

Have you modified the DART code?

Yes mom6-z branch because the restart files have lz (see #906)

Build information

Please describe:

1. Derecho
2. Intel

Also the namelist options 'single_file_out' for perfect_model_obs are confusing for users (and developers).

Start by reproducing perfect_model_obs with write_output_state_to_file = .false. and single_file_out = .true., then trace the namelist handling and output setup. The fix is done when no true_state.nc is written while obs_seq.out is still produced, including for the reported MOM6 case.