Add night lights composites for ABI, AHI and AMI

[simonrp84]

This PR adds true color composites that use the VIIRS night lights as a background for night-time pixels. No code changes have been made, it simply re-uses the existing SEVIRI night lights composite (natural_color_with_night_ir). In the case of the new instruments, however, I chose to use true_color instead of natural_color to reflect the capabilities of the instruments.

• Tests passed

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[coveralls]

Coverage increased (+0.2%) to 89.804% when pulling 2c48c3e on simonrp84:night_lights_comps into 446af3a on pytroll:master.

[codecov]

Codecov Report

Merging #1141 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master    #1141   +/-   ##
=======================================
  Coverage   89.60%   89.60%           
=======================================
  Files         200      200           
  Lines       29484    29484           
=======================================
  Hits        26420    26420           
  Misses       3064     3064           

Impacted Files	Coverage Δ
satpy/readers/__init__.py	95.09% <0.00%> (ø)

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[simonrp84]

I've just pushed a few more composites to this, which add a _nocorr version of true_color and natural_color for SEVIRI, ABI, AMI and AHI.
These versions (i.e: true_color_nocorr) disable both the solar zenith and rayleigh scattering corrections, and hence produce data as it looked to the satellite sensor.

[codecov]

Codecov Report

Merging #1141 into master will increase coverage by 0.19%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1141      +/-   ##
==========================================
+ Coverage   89.60%   89.80%   +0.19%     
==========================================
  Files         200      204       +4     
  Lines       29484    30148     +664     
==========================================
+ Hits        26420    27074     +654     
- Misses       3064     3074      +10     

Impacted Files	Coverage Δ
satpy/readers/tropomi_l2.py	87.06% <0.00%> (-2.94%)	⬇️
satpy/readers/avhrr_l1b_gaclac.py	93.33% <0.00%> (-1.34%)	⬇️
satpy/readers/viirs_compact.py	64.12% <0.00%> (-1.27%)	⬇️
satpy/readers/hrit_jma.py	97.94% <0.00%> (-1.22%)	⬇️
satpy/tests/test_composites.py	99.88% <0.00%> (-0.12%)	⬇️
satpy/writers/cf_writer.py	93.77% <0.00%> (-0.03%)	⬇️
satpy/writers/geotiff.py	90.74% <0.00%> (ø)
satpy/tests/test_dataset.py	100.00% <0.00%> (ø)
satpy/readers/netcdf_utils.py	100.00% <0.00%> (ø)
satpy/readers/seviri_l1b_native.py	71.96% <0.00%> (ø)
... and 32 more

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[pnuu]

Couple things came to mind, but otherwise looks good to me.

1. The composites in <instrument>.yaml should IMHO use the channel names, and only the generic ones in visir.yaml have wavelengths. This is debatable, so not strictly necessary here. But it would make things look more consistent and ensure the correct channels to be used.
2. As the _nocorr composites use reflectances, correction for the Sun-Earth distance should be needed as an modifier. We don't have that modifier yet, and it might need some reader-level metadata additions and checks, so not a problem here. For reference on reflectance/correction/modifier discussion: When is reflectance actually reflectance? #536

[simonrp84]

I've now, after a few failures, updated the YAML with @pnuu's request - the channels needed for the new composites are now specified by name.

[pnuu]

Added two suggested changes (one of which doesn't work with the current version). Otherwise LGTM.