Add xarray example

[ppinchuk]

Add README and notebook showing some xarray + rex usage. README is probably easier to review here: https://github.com/NREL/rex/tree/pp/xarray_example/examples/xarray (similar for the notebook).

Could also build the docs locally but that's more work :)

reVX tests are broken since this version of rex uses numpy > 2.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 67.72%. Comparing base (5522a2e) to head (76abac0).

Additional details and impacted files

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##             main     #208   +/-   ##
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  Coverage   67.72%   67.72%           
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  Files          40       40           
  Lines        7090     7090           
  Branches     1037     1037           
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  Hits         4802     4802           
  Misses       1961     1961           
  Partials      327      327           

Flag	Coverage Δ
unittests	67.72% <ø> (ø)

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