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.

• Tests added
• Tests passed: Passes pytest pyspectral
• Passes flake8 pyspectral
• Fully documented

[codecov]

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 ⚠️

Additional details and impacted files

@@            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	Coverage Δ
unittests	90.00% <99.00%> (+0.72%)	⬆️

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[pnuu]

LGTM.

I love how much you could just delete from the original code 😂

[pnuu]

Closes #201

[pnuu]

Closes pytroll/satpy#2613