Add default value in vectorize when dataarray has no name, and a warning

[Imanflow]

• Closes Setting a name for the value column in vectorize #164
• Tests added
• Fully documented, including docs/history.rst for all changes and docs/geocube.rst for new API

[snowman2]

Running this should help address linting issues:

python -m pip install pre-commit
cd ./geocube
pre-commit install
pre-commit run -a

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.28%. Comparing base (a69f63d) to head (c06e32b).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
+ Coverage   96.23%   96.28%   +0.04%     
==========================================
  Files          14       14              
  Lines         452      457       +5     
==========================================
+ Hits          435      440       +5     
  Misses         17       17              

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[snowman2]

Thanks @Imanflow 👍