Add warning for SEVIRI native reader in case of bad data

[simonrp84]

On occasion, a SEVIRI native file contains bad data in which the digital counts in the file are invalid. This is not picked up by the reader as there's no quality checking.

This PR adds a check for the QQOV flag in the header. In cases where it is NOK a warning is raised to alert the user that the file is likely to contain bad quality data.

• Tests added

[codecov]

Codecov Report

Merging #2198 (f22274d) into main (a5573c1) will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #2198      +/-   ##
==========================================
+ Coverage   94.01%   94.07%   +0.05%     
==========================================
  Files         289      290       +1     
  Lines       44614    44667      +53     
==========================================
+ Hits        41945    42021      +76     
+ Misses       2669     2646      -23     

Flag	Coverage Δ
behaviourtests	4.72% <0.00%> (-0.02%)	⬇️
unittests	94.73% <100.00%> (+0.05%)	⬆️

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

Impacted Files	Coverage Δ
satpy/readers/seviri_l1b_native_hdr.py	100.00% <ø> (ø)
satpy/readers/seviri_l1b_native.py	86.36% <100.00%> (+0.11%)	⬆️
satpy/tests/reader_tests/test_seviri_l1b_native.py	100.00% <100.00%> (ø)
satpy/utils.py	24.69% <0.00%> (-0.31%)	⬇️
satpy/tests/reader_tests/test_viirs_compact.py	100.00% <0.00%> (ø)
satpy/tests/test_cf_roundtrip.py	100.00% <0.00%> (ø)
satpy/tests/test_writers.py	98.99% <0.00%> (+0.02%)	⬆️
satpy/tests/test_readers.py	98.95% <0.00%> (+0.04%)	⬆️
satpy/readers/satpy_cf_nc.py	97.80% <0.00%> (+0.04%)	⬆️
satpy/readers/yaml_reader.py	97.50% <0.00%> (+0.22%)	⬆️
... and 6 more

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[coveralls]

Coverage increased (+0.002%) to 94.682% when pulling f22274d on simonrp84:native_baddata_flag2 into 687494c on pytroll:main.

[mraspaud]

Looks good, thanks for adding this. Just a couple of things.

[simonrp84]

Both your comments should be addressed now @mraspaud

[mraspaud]

LGTM