Fix AMI geolocation being off by 1 pixel

[djhoese]

I noticed when working with AMI L1b data that the geolocation for a full disk image was not square. Turns out I had a +/- bug in the math for the area extents. This PR fixes this.

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

CC @simonrp84

[codecov]

Codecov Report

Merging #1034 into master will decrease coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master    #1034      +/-   ##
==========================================
- Coverage   87.27%   87.26%   -0.01%     
==========================================
  Files         188      188              
  Lines       28739    28735       -4     
==========================================
- Hits        25082    25077       -5     
- Misses       3657     3658       +1

Impacted Files	Coverage Δ
satpy/tests/reader_tests/test_ami_l1b.py	94.73% <ø> (ø)	⬆️
satpy/readers/_geos_area.py	100% <100%> (ø)	⬆️
satpy/readers/ami_l1b.py	97.39% <100%> (-0.09%)	⬇️
satpy/scene.py	89.63% <0%> (-0.18%)	⬇️

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[coveralls]

Coverage decreased (-0.005%) to 87.27% when pulling c58223c on djhoese:bugfix-ami-geo into c80a510 on pytroll:master.

[mraspaud]

LGTM