Add support for zero-copy conversions from an xarray to a DimArray

[JamesWrigley]

In the process I also refactored the code to rely on PythonCall more, which simplified it quite a lot. One thorny issue is that this could be considered a breaking change since the parent array type changed from Array to PyArray, which is not a DenseArray (mentioned in the new docs). But I feel that the parent array type is a pretty internal thing to change, and since in the worst case it won't break any existing code but just make it slower, maybe we can still put it in a patch release?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 83.68%. Comparing base (20b42df) to head (23cb84b).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #972      +/-   ##
==========================================
+ Coverage   83.44%   83.68%   +0.23%     
==========================================
  Files          54       54              
  Lines        5154     5129      -25     
==========================================
- Hits         4301     4292       -9     
+ Misses        853      837      -16     

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