Add LAC support to the avhrr-gac-lac reader

[mraspaud]

The newest release of pygac should support LAC data reading. This PR makes sure satpy does too.

• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy
• Fully documented

[coveralls]

Coverage increased (+0.0008%) to 86.895% when pulling 28d5654 on mraspaud:feature-lac into 7219160 on pytroll:master.

[codecov]

Codecov Report

Merging #994 into master will increase coverage by 0.01%.
The diff coverage is 85.18%.

@@            Coverage Diff             @@
##           master     #994      +/-   ##
==========================================
+ Coverage   86.89%   86.91%   +0.01%     
==========================================
  Files         181      181              
  Lines       27500    27778     +278     
==========================================
+ Hits        23896    24143     +247     
- Misses       3604     3635      +31

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_avhrr_l1b_gaclac.py	98.03% <100%> (+0.04%)	⬆️
satpy/readers/avhrr_l1b_gaclac.py	94.59% <77.77%> (-0.26%)	⬇️
satpy/tests/reader_tests/test_seviri_l1b_native.py	93.51% <0%> (+0.18%)	⬆️
satpy/readers/seviri_l1b_native.py	73.87% <0%> (+6.63%)	⬆️

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[sfinkens]

Good work, LGMT!