ENH: Added band_as_variable option to open_rasterio

[snowman2]

• [x] Closes #296
• [x] Closes #81
• [x] Tests added
• [x] Fully documented, including docs/history.rst for all changes and docs/rioxarray.rst for new API

[codecov[bot]]

Codecov Report

Base: 96.16% // Head: 96.33% // Increases project coverage by +0.17% :tada:

Coverage data is based on head (6dc872c) compared to base (104608b). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #600 +/- ## ========================================== + Coverage 96.16% 96.33% +0.17% ========================================== Files 11 11 Lines 1641 1691 +50 ========================================== + Hits 1578 1629 +51 + Misses 63 62 -1 ``` | [Impacted Files](https://codecov.io/gh/corteva/rioxarray/pull/600?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=corteva) | Coverage Δ | | |---|---|---| | [rioxarray/\_io.py](https://codecov.io/gh/corteva/rioxarray/pull/600/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=corteva#diff-cmlveGFycmF5L19pby5weQ==) | `96.44% <100.00%> (+0.60%)` | :arrow_up: | Help us with your feedback. Take ten seconds to tell us [how you rate us](https://about.codecov.io/nps?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=corteva). Have a feature suggestion? [Share it here.](https://app.codecov.io/gh/feedback/?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=corteva)

:umbrella: View full report at Codecov.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.

[snowman2]

Thanks @clausmichele @justingruca :+1: