BUG: Handle netCDF _Unsigned and load in all attributes

[snowman2]

• [x] Closes #574
• Related: #525 & #573
• [x] Tests added
• [x] Fully documented, including docs/history.rst for all changes and docs/rioxarray.rst for new API

Updated unsigned logic based on: https://github.com/pydata/xarray/blob/abe1e613a96b000ae603c53d135828df532b952e/xarray/coding/variables.py#L306-L334

This required updating loading attributes to ensure the _Unsigned property is saved/loaded correctly.

[codecov[bot]]

Codecov Report

Merging #575 (170bac9) into master (f23d3d4) will increase coverage by 0.05%. The diff coverage is 97.33%.

@@            Coverage Diff             @@
##           master     #575      +/-   ##
==========================================
+ Coverage   96.25%   96.30%   +0.05%     
==========================================
  Files          13       13              
  Lines        1603     1652      +49     
==========================================
+ Hits         1543     1591      +48     
- Misses         60       61       +1     

Impacted Files	Coverage Δ
rioxarray/_io.py	96.00% <96.92%> (+0.14%)	:arrow_up:
rioxarray/raster_writer.py	95.32% <100.00%> (+0.42%)	:arrow_up:

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[snowman2]

Thanks @justingruca :+1: