Convert ahi hsd reader to dask and xarray

[mraspaud]

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

[codecov-io]

Codecov Report

Merging #274 into develop will increase coverage by 0.14%.
The diff coverage is 23.33%.

@@             Coverage Diff             @@
##           develop     #274      +/-   ##
===========================================
+ Coverage    63.93%   64.07%   +0.14%     
===========================================
  Files          114      114              
  Lines        13963    13932      -31     
===========================================
  Hits          8927     8927              
+ Misses        5036     5005      -31

Impacted Files	Coverage Δ
satpy/readers/ahi_hsd.py	22.16% <23.33%> (+3.18%)	⬆️

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[coveralls]

Coverage increased (+0.1%) to 64.076% when pulling 104aff0 on feature-ahi-dask into f4445e9 on develop.