Fix hrit goes to support python 3

[mraspaud]

• Closes HRIT GOES reader is broken #165
• Tests added
• Tests passed
• Passes git diff origin/master **/*py | flake8 --diff
• Fully documented

[coveralls]

Coverage remained the same at 65.901% when pulling e106075 on fix-hrit-goes-py3 into e49c634 on master.

[codecov]

Codecov Report

Merging #303 into master will not change coverage.
The diff coverage is 0%.

@@          Coverage Diff           @@
##           master    #303   +/-   ##
======================================
  Coverage    65.9%   65.9%           
======================================
  Files         118     118           
  Lines       14332   14332           
======================================
  Hits         9445    9445           
  Misses       4887    4887

Impacted Files	Coverage Δ
satpy/readers/hrit_goes.py	50.68% <0%> (ø)	⬆️

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