feat: ability to save/read directly from an open file

[steven-murray]

Fixes #170

Adds ability to use an open hdf5 file/group on .save and .read.

[codecov[bot]]

Codecov Report

Base: 86.31% // Head: 86.54% // Increases project coverage by +0.22% :tada:

Coverage data is based on head (ae8f13b) compared to base (7e05226). Patch coverage: 94.91% of modified lines in pull request are covered.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #288 +/- ## ========================================== + Coverage 86.31% 86.54% +0.22% ========================================== Files 13 13 Lines 2784 2809 +25 ========================================== + Hits 2403 2431 +28 + Misses 381 378 -3 ``` | [Impacted Files](https://codecov.io/gh/21cmfast/21cmFAST/pull/288?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=21cmfast) | Coverage Δ | | |---|---|---| | [src/py21cmfast/\_utils.py](https://codecov.io/gh/21cmfast/21cmFAST/pull/288/diff?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=21cmfast#diff-c3JjL3B5MjFjbWZhc3QvX3V0aWxzLnB5) | `88.90% <94.91%> (+0.82%)` | :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=21cmfast). 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=21cmfast)

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