REF: Store grid_mapping in encoding instead of attrs

[snowman2]

• Closes Add full support for xarray decode_coords #282
• Tests added
• Fully documented, including docs/history.rst for all changes and docs/rioxarray.rst for new API

Had to add DEP: Python 3.7+ and xarray 0.17+ for this to be safe to use.

• Closes Python 3.6 Support #215

[codecov]

Codecov Report

Merging #284 (596be53) into master (0b32bd0) will decrease coverage by 0.25%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #284      +/-   ##
==========================================
- Coverage   93.94%   93.68%   -0.26%     
==========================================
  Files          12       12              
  Lines        1354     1362       +8     
==========================================
+ Hits         1272     1276       +4     
- Misses         82       86       +4     

Impacted Files	Coverage Δ
rioxarray/__init__.py	100.00% <100.00%> (+18.18%)	⬆️
rioxarray/_io.py	85.98% <100.00%> (-1.69%)	⬇️
rioxarray/raster_array.py	98.25% <100.00%> (ø)
rioxarray/rioxarray.py	97.03% <100.00%> (+0.09%)	⬆️

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