Add initial Scene/DataTree conversion via Scene.to_xarray_datatree

[djhoese]

Related to #2352, I thought I'd add a Scene.to_xarray_datatree method to the Scene. I want to use this as a stepping stone into more DataTree usage in Satpy and see where it takes us. I also wanted a reason to play around with DataTree more and see how it goes.

At the time of writing this is a pretty simple implementation, but also provides a lot of flexibility. When discussing this concept in #2352 I brought up the ideas of "schemes" or predefined tree-structuring methods that users could choose from. For example, "area"-based or "DataID"-based organizing. As I started working on this (TDD for the win!) I realized I could just structure it as "what metadata keys should I use for each 'group'". This means users can select the level of the hierarchy of the tree. For example, group by platform_name and sensor by specifying group_keys=("platform_name", "sensor").

So at this point, this works as described above, but I'm looking for feedback for others. I seem to remember @mraspaud and @gerritholl being part of this conversation and @ghiggi doing some reworking of Scene.to_xarray_dataset related to CF compatibility. I'm curious what you guys think of this.

There is still a lot TODO like:

• handling grouping by area. For example, what do I use as the node name? area hash? Area name (if it exists, pyresample 2.0 will have optional names)?.
• handling of other non-string properties including resolution (units?), calibration enum, etc.
• What attributes to define on each group
• Documentation

• Closes #xxxx
• Tests added
• Fully documented

[codecov]

Codecov Report

Merging #2452 (51c389d) into main (087acd3) will increase coverage by 0.05%.
The diff coverage is 93.24%.

@@            Coverage Diff             @@
##             main    #2452      +/-   ##
==========================================
+ Coverage   94.75%   94.81%   +0.05%     
==========================================
  Files         335      340       +5     
  Lines       48852    49417     +565     
==========================================
+ Hits        46291    46853     +562     
- Misses       2561     2564       +3     

Flag	Coverage Δ
behaviourtests	4.41% <1.35%> (+<0.01%)	⬆️
unittests	95.43% <93.24%> (+0.04%)	⬆️

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

Impacted Files	Coverage Δ
satpy/_scene_converters.py	91.42% <91.42%> (ø)
satpy/tests/scene_tests/test_conversions.py	97.29% <94.44%> (-2.71%)	⬇️
satpy/scene.py	93.54% <100.00%> (+0.02%)	⬆️

... and 75 files with indirect coverage changes

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[coveralls]

Coverage: 95.38% (-0.003%) from 95.383% when pulling 51c389d on djhoese:feature-scene-to-datatree into 5310f1a on pytroll:main.

[BENR0]

I think that this could also be really useful for #2171. Currently the sorting is done by platform/sensor and the by area because this made the most sense when I started. But I think it could be useful to display the Scene by a different hierarchy. So this would make that task easier I think.