Update scale factor and offset [viirs_l1b]

[zxdawn]

The scale factor and offset should be updated for BT data. The easiest way seems to be updating these parameters using the factor output.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.07%. Comparing base (5984c29) to head (dd11b1e).
Report is 5 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3016      +/-   ##
==========================================
- Coverage   96.10%   96.07%   -0.03%     
==========================================
  Files         377      377              
  Lines       55163    55175      +12     
==========================================
- Hits        53012    53011       -1     
- Misses       2151     2164      +13     

Flag	Coverage Δ
behaviourtests	3.93% <0.00%> (-0.01%)	⬇️
unittests	96.17% <100.00%> (-0.03%)	⬇️

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[coveralls]

Pull Request Test Coverage Report for Build 12583702887

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
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• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 12 of 12 (100.0%) changed or added relevant lines in 2 files are covered.
• 8 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-0.01%) to 96.193%

---

Files with Coverage Reduction	New Missed Lines	%
satpy/tests/utils.py	2	93.16%
satpy/tests/reader_tests/gms/test_gms5_vissr_l1b.py	3	98.67%
satpy/tests/reader_tests/gms/test_gms5_vissr_navigation.py	3	97.18%

Totals
Change from base Build 12299617024:	-0.01%
Covered Lines:	53260
Relevant Lines:	55368

---

💛 - Coveralls

[djhoese]

I think I'm OK merging this, but I just realized that typically I advise things like _FillValue and scale_factor/add_offset to be removed in readers because they are misleading or confusing in later parts of Satpy processing. For example, the factor/offset here are likely for 16-bit integers right? But we don't know if that is signed or unsigned. Or it could be for 8-bit or 32-bit integers, we don't know. Or someone could want to save to a NetCDF 8-bit integer and use these factors and offsets and get incompatible scaled values (out of range).

Additionally, these are being added to ".attrs". Should they maybe be put in .encoding?

[zxdawn]

Thanks for the idea! I see the factors are applied for all variables:

        factors = (scale_factor, scale_offset)
        factors = self.adjust_scaling_factors(factors, metadata["file_units"], ds_info.get("units"))
        if factors[0] != 1 or factors[1] != 0:
            data *= factors[0]
            data += factors[1]

The easiest way may be removing these not-updated attrs (if they exist):

        # remove scale_factor and scale_offset
        data.attrs.pop("scale_factor", None)
        data.attrs.pop("add_offset", None)

[djhoese]

I think that makes sense to me.