create dir before writing to netcdf

[aurelienfalco]

Minor fix to avoid having to create the directory by hand when creating a netcdf

[codecov]

Codecov Report

❌ Patch coverage is 60.00000% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 82.12%. Comparing base (c94910b) to head (dde1be0).

Files with missing lines	Patch %	Lines
xdas/core/dataarray.py	66.66%	1 Missing ⚠️
xdas/core/datacollection.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #51      +/-   ##
==========================================
- Coverage   82.15%   82.12%   -0.03%     
==========================================
  Files          36       36              
  Lines        4091     4096       +5     
==========================================
+ Hits         3361     3364       +3     
- Misses        730      732       +2     

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