Update AHI HSD calibration coefficients

[simonrp84]

Currently, the AHI HSD reader uses the default calibration coefficients for VIS channels: gain_count2rad_conversion and offset_count2rad_conversion in Block 5 of the header.
However, updated calibration coefficients are available in Block 6 of the header: cali_gain_count2rad_conversion and cali_offset_count2rad_conversion.

This PR switches from using the old coefficients by default to using the new ones by default.

In addition, this PR removes the clipping of negative radiances. In the current main branch any radiances below zero are automatically set to zero. This is undesirable as negative radiances are useful for analysing sensor noise, amongst other things, so I remove this unnecessary clipping.
There is also clipping for the reflectance values, but I've left that in for further discussion.

[mraspaud]

Tests are failing, otherwise lgtm

[sfinkens]

Thanks, makes sense to use the latest coefficients! Can you please also update in the docs that this is the new default?

Regarding radiance clipping: Although I agree that clipping should at least be optional, maybe we should wait for a decision in #1911?

[codecov]

Codecov Report

Merging #1908 (3b3cd59) into main (85656a8) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #1908      +/-   ##
==========================================
- Coverage   93.41%   93.41%   -0.01%     
==========================================
  Files         275      275              
  Lines       40732    40730       -2     
==========================================
- Hits        38048    38046       -2     
  Misses       2684     2684              

Flag	Coverage Δ
behaviourtests	4.82% <0.00%> (+<0.01%)	⬆️
unittests	93.97% <100.00%> (-0.01%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_ahi_hsd.py	100.00% <ø> (ø)
satpy/readers/ahi_hsd.py	97.29% <100.00%> (ø)
satpy/readers/fci_l1c_nc.py	97.93% <0.00%> (-0.05%)	⬇️
satpy/tests/reader_tests/test_fci_l1c_nc.py	100.00% <0.00%> (ø)

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[coveralls]

Coverage decreased (-0.0003%) to 93.905% when pulling 3b3cd59 on simonrp84:ahi_calib into 85656a8 on pytroll:main.

[simonrp84]

Ok, it seems that the tests are complaining about coverage in an unaffected file. Odd. Anyway, AHI coverage remains the same.

Thanks for the pointer about documentation @sfinkens - I've updated the docs now.

[mraspaud]

LGTM