Allow skipping unit conversion in NinJoTIFF

[gerritholl]

Add an option to the NinJoTIFF writer to skip unit conversion. For a
detailed rationale, see #2024.

• Closes Allow to skip unit conversion in ninjotiff writer #2024
• Tests added
• Fully documented

[codecov]

Codecov Report

Merging #2025 (0ab9806) into main (82112a6) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #2025   +/-   ##
=======================================
  Coverage   93.56%   93.56%           
=======================================
  Files         282      282           
  Lines       41637    41650   +13     
=======================================
+ Hits        38957    38970   +13     
  Misses       2680     2680           

Flag	Coverage Δ
behaviourtests	4.78% <0.00%> (-0.01%)	⬇️
unittests	94.09% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
satpy/tests/writer_tests/test_ninjotiff.py	100.00% <100.00%> (ø)
satpy/writers/ninjotiff.py	80.64% <100.00%> (+0.98%)	⬆️

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[coveralls]

Coverage increased (+0.002%) to 94.023% when pulling 0ab9806 on gerritholl:ninjotiff-skip-unit-conversion into 82112a6 on pytroll:main.

[djhoese]

I definitely shouldn't be the final word on this, but this seems like a reasonable solution. I think the general confusion that does/will arise for anyone "out of the know" trying to compare enhancements and data values will be happy with this and with stretch limits being specified in the units of the input data. I think there are other options (don't remember if you mentioned this in the related issue) that would require some duplicate code to reimplement save_datasets or maybe even a custom Enhancer object for the writer so that the unit conversion can happen after the initial Kelvin-based stretch.