BUG: Fix closing files manually

[snowman2]

In #606 it became evident that the files were not being closed. This fix should help users to close Datasets that have data_vars pointing to files.

• [x] Related #485
• [x] Related #544
• [x] Fully documented, including docs/history.rst for all changes and docs/rioxarray.rst for new API

[codecov[bot]]

Codecov Report

Base: 96.33% // Head: 96.37% // Increases project coverage by +0.04% :tada:

Coverage data is based on head (a85545d) compared to base (49f2815). Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #607 +/- ## ========================================== + Coverage 96.33% 96.37% +0.04% ========================================== Files 11 11 Lines 1691 1711 +20 ========================================== + Hits 1629 1649 +20 Misses 62 62 ``` | [Impacted Files](https://codecov.io/gh/corteva/rioxarray/pull/607?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/607/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=corteva#diff-cmlveGFycmF5L19pby5weQ==) | `96.55% <100.00%> (+0.10%)` | :arrow_up: | | [rioxarray/xarray\_plugin.py](https://codecov.io/gh/corteva/rioxarray/pull/607/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=corteva#diff-cmlveGFycmF5L3hhcnJheV9wbHVnaW4ucHk=) | `92.59% <100.00%> (+1.28%)` | :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)

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[snowman2]

Thanks @justingruca :+1: