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

[Imanflow]

• [x] Closes #164
• [x] Tests added
• [x] 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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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 :+1: