prepare for legacy output file deprecation with future version 2.2.

[amoodie]

The behavior of the legacy NetCDF output format option, and the new file output format is expected to change in version 2.2.0.

With v2.2.0 the default output file comply with the sandsuet data specification, and the legacy_output=True` option will output the current configuration. The core data will not change with v2.2, but the names and attributes of components of the data output is expected change.

I don't expect dropping the current legacy option to affect any users. Current users will be able to maintain the current file format with legacy_netcdf=True for versions moving forward from v2.2. They will be encouraged to adapt scripts to the new format, but we will maintain backwards compatibility.

See sandsuet data specification for expected file format.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 79.10%. Comparing base (8a4ab35) to head (80c0dde).
⚠️ Report is 12 commits behind head on develop.

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #293      +/-   ##
===========================================
+ Coverage    79.00%   79.10%   +0.10%     
===========================================
  Files           12       12              
  Lines         2643     2637       -6     
===========================================
- Hits          2088     2086       -2     
+ Misses         555      551       -4     

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