Add hashing to StackedAreaDefinitions

[mraspaud]

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

[coveralls]

Coverage decreased (-0.05%) to 87.352% when pulling 2525611 on feature-hash-stacked-areas into 82d5544 on master.

[codecov]

Codecov Report

Merging #119 into master will decrease coverage by 0.04%.
The diff coverage is 25%.

@@            Coverage Diff             @@
##           master     #119      +/-   ##
==========================================
- Coverage   87.39%   87.35%   -0.05%     
==========================================
  Files          34       34              
  Lines        5491     5495       +4     
==========================================
+ Hits         4799     4800       +1     
- Misses        692      695       +3

Impacted Files	Coverage Δ
pyresample/geometry.py	75.33% <25%> (-0.27%)	⬇️

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[djhoese]

I'm marking this as a bug so it shows up in the release notes (via loghub) as such.