Allow dataarrays and preserve dtype in rad2temp

[mraspaud]

This PR allow xarray's dataarrays to be passe to radiance to temperature conversions, and also makes these functions preserve dtype.

• [x] Tests added
• [x] Tests passed: Passes pytest pyspectral
• [x] Passes flake8 pyspectral
• [x] Fully documented

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (fd32fe9) 89.27% compared to head (ec59df7) 90.00%.

Files	Patch %	Lines
pyspectral/utils.py	96.15%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #203 +/- ## ========================================== + Coverage 89.27% 90.00% +0.72% ========================================== Files 23 23 Lines 2545 2580 +35 ========================================== + Hits 2272 2322 +50 + Misses 273 258 -15 ``` | [Flag](https://app.codecov.io/gh/pytroll/pyspectral/pull/203/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | Coverage Δ | | |---|---|---| | [unittests](https://app.codecov.io/gh/pytroll/pyspectral/pull/203/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll) | `90.00% <99.00%> (+0.72%)` | :arrow_up: | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=pytroll#carryforward-flags-in-the-pull-request-comment) to find out more.

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[pnuu]

Closes https://github.com/pytroll/pyspectral/issues/201

[pnuu]

Closes https://github.com/pytroll/satpy/issues/2613