Import DataTree from xarray

[mraspaud]

This PR fixes the error with the unstable build hopefully.
Note that very shortly, this will need to be changed again as DataTree will become public API in xarray.
Progress in pydata/xarray#8572

• Closes #xxxx
• Tests added
• Fully documented
• Add your name to AUTHORS.md if not there already

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.06%. Comparing base (692c9b1) to head (044a0ec).
Report is 26 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2926      +/-   ##
==========================================
+ Coverage   96.05%   96.06%   +0.01%     
==========================================
  Files         373      373              
  Lines       54479    54477       -2     
==========================================
+ Hits        52330    52336       +6     
+ Misses       2149     2141       -8     

Flag	Coverage Δ
behaviourtests	3.99% <0.00%> (+<0.01%)	⬆️
unittests	96.16% <100.00%> (+0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

[coveralls]

Pull Request Test Coverage Report for Build 11290671566

Details

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.02%) to 96.166%

---

Totals
Change from base Build 11250781903:	0.02%
Covered Lines:	52570
Relevant Lines:	54666

---

💛 - Coveralls