Update SLSTR composites

[simonrp84]

This PR adds:

1. A new natural_color composite for SLSTR that uses the 0.6, 0.8 and 1.6 micron bands.
2. Composites for the oblique view. natural_color and overview. Previously, overview was for nadir only.

This only changes the yaml file, no changes to the python code are needed.

[mraspaud]

Thanks for the PR. @djhoese did we decide that overview and natural_colors should be sunz corrected by default ?

[djhoese]

Yes I think so. sunz if possible, rayleigh correction if possible. At least that's what we've done for VIIRS, ABI, and I think SEVIRI.

[coveralls]

Coverage increased (+0.03%) to 88.928% when pulling 57b2675 on simonrp84:slstr-composites into ca8595f on pytroll:master.

[codecov]

Codecov Report

Merging #1082 into master will increase coverage by 0.04%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master    #1082      +/-   ##
==========================================
+ Coverage   88.89%   88.94%   +0.04%     
==========================================
  Files         192      194       +2     
  Lines       29657    29823     +166     
==========================================
+ Hits        26364    26525     +161     
- Misses       3293     3298       +5

Impacted Files	Coverage Δ
satpy/readers/slstr_l1b.py	63.94% <25%> (+0.85%)	⬆️
satpy/readers/tropomi_l2.py	94.11% <0%> (-1.01%)	⬇️
satpy/scene.py	89.73% <0%> (-0.18%)	⬇️
satpy/tests/reader_tests/test_iasi_l2_so2_bufr.py	96.15% <0%> (ø)
satpy/readers/iasi_l2_so2_bufr.py	94.44% <0%> (ø)
satpy/tests/reader_tests/__init__.py	98.5% <0%> (+0.02%)	⬆️
satpy/writers/ninjotiff.py	60% <0%> (+0.62%)	⬆️
satpy/tests/writer_tests/test_ninjotiff.py	100% <0%> (+2.17%)	⬆️

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