Fix striding slicing in AreaDefinition

[djhoese]

I noticed that slicing an AreaDefinition with [::4, ::4] I was getting unexpected sizes like a 6000x10000 area becoming a 1499x2499 area. This PR fixes this off-by-one issue.

• Tests added
• Tests passed
• Passes git diff origin/master **/*py | flake8 --diff

[codecov]

Codecov Report

Merging #193 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #193      +/-   ##
==========================================
+ Coverage   86.71%   86.71%   +<.01%     
==========================================
  Files          37       37              
  Lines        6818     6821       +3     
==========================================
+ Hits         5912     5915       +3     
  Misses        906      906

Impacted Files	Coverage Δ
pyresample/geometry.py	84.84% <100%> (+0.03%)	⬆️
pyresample/test/test_geometry.py	97.08% <100%> (ø)	⬆️

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[coveralls]

Coverage increased (+0.006%) to 86.688% when pulling e77285d on djhoese:bugfix-strided-area-slice into 5ddced8 on pytroll:master.

[coveralls]

Coverage increased (+0.006%) to 86.688% when pulling e77285d on djhoese:bugfix-strided-area-slice into 5ddced8 on pytroll:master.

[mraspaud]

LGTM