Add function to the SLSTR L1 reader to enable correction of VIS radiances.

[simonrp84]

The SLSTR instrument is producing data with significant bias in the visible channels.
As defined in the currently-valid product notice, the S5 and S6 channel radiances should be adjusted by between 12 and 20% depending on channel and view. Corrections are also needed for other VIS channels, and these correction factors will be available soon.

In the meantime, this PR enables users to load SLSTR data with the currently-valid radiance correction factors applied, which will produce better data for users requiring the S5 or S6 radiances.

• Tests added
• Tests passed
• Passes flake8 satpy
• Fully documented

[codecov]

Codecov Report

Merging #1401 into master will increase coverage by 0.06%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1401      +/-   ##
==========================================
+ Coverage   90.56%   90.62%   +0.06%     
==========================================
  Files         228      229       +1     
  Lines       33406    33676     +270     
==========================================
+ Hits        30254    30519     +265     
- Misses       3152     3157       +5     

Impacted Files	Coverage Δ
satpy/readers/slstr_l1b.py	75.56% <100.00%> (+12.04%)	⬆️
satpy/tests/reader_tests/test_slstr_l1b.py	100.00% <100.00%> (ø)
satpy/readers/mimic_TPW2_nc.py	80.95% <0.00%> (-9.41%)	⬇️
satpy/scene.py	90.42% <0.00%> (-0.18%)	⬇️
satpy/tests/reader_tests/test_mimic_TPW2_nc.py	95.94% <0.00%> (-0.06%)	⬇️
satpy/enhancements/mimic.py	0.00% <0.00%> (ø)
satpy/tests/test_multiscene.py	99.42% <0.00%> (ø)
satpy/readers/seviri_l2_bufr.py	93.90% <0.00%> (ø)
satpy/readers/seviri_l2_grib.py	86.45% <0.00%> (ø)
satpy/tests/reader_tests/test_mimic_TPW2_lowres.py	97.58% <0.00%> (ø)
... and 8 more

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[coveralls]

Coverage increased (+0.03%) to 90.596% when pulling 08b1306 on simonrp84:slstr_viscal into 12917ab on pytroll:master.

[ghost]

DeepCode's analysis on #08b130 found:

• ℹ️ 1 minor issue. 👇

Top issues

Description	Example fixes
Access to a protected member _cal_rad of a client class Occurrences: test_slstr_l1b.py:234	🔧 Example fixes

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[simonrp84]

This should be ready for review now.
Deepcode is moaning about my test of a protected function. Any ideas what to do about that? Initially I tried to test inside the calibration routine itself (i.e: the da.map_blocks call) but that didn't work well as there's so many things that need mocking for the call - which I don't have the time to figure out at present.
Thoughts welcome!

[mraspaud]

LGTM!