note on netcdf 64bit vs netcdf-4 file creation DART

noticed this when testing:

direct_netcdf_mod.f90 creates netcdf files in 64bit offset mode:
https://github.com/NCAR/DART/blob/62ecfa2a2b4c7de55f98f434de49e076060fd82c/assimilation_code/modules/io/direct_netcdf_mod.f90#L1617

netcdf_utilities_mod.f90 creates netcdf files in classic mode
https://github.com/NCAR/DART/blob/62ecfa2a2b4c7de55f98f434de49e076060fd82c/assimilation_code/modules/utilities/netcdf_utilities_mod.f90#L2409

Do we care about this? I don't know. Just making a note.

running perfect_model_obs:
input state file is netCDF-4 time as unlimited
dart created file from perfect_model_obs has a time dimension but not unlimited. 64-bit offset.

If you create the dart file as netCDF-4 (same as the template file) you get

message: nf90_enddef end define mode: NetCDF: One or more variable sizes viola
 te format constraints

so the 64 bit offset is getting around not having the unlimited dimension.

Originally posted by @hkershaw-brown in https://github.com/NCAR/DART/issues/278#issuecomment-917251539

Compare the file-creation paths in assimilation_code/modules/io/direct_netcdf_mod.f90 around line 1617 and assimilation_code/modules/utilities/netcdf_utilities_mod.f90 around line 2409. Run perfect_model_obs with a netCDF-4 input and reproduce the nf90_enddef format-constraint message. Done should establish the intended creation mode and resolve or document the mismatch.