Adapt slstr reader to xarray&dask

[mraspaud]

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

[coveralls]

Coverage decreased (-0.02%) to 69.031% when pulling d153a5a on fix-slstr into ac8bdde on master.

[codecov]

Codecov Report

Merging #331 into master will decrease coverage by 0.02%.
The diff coverage is 13.33%.

@@            Coverage Diff             @@
##           master     #331      +/-   ##
==========================================
- Coverage   69.05%   69.03%   -0.03%     
==========================================
  Files         123      123              
  Lines       15601    15609       +8     
==========================================
+ Hits        10773    10775       +2     
- Misses       4828     4834       +6

Impacted Files	Coverage Δ
satpy/readers/nc_slstr.py	23.96% <13.33%> (+0.07%)	⬆️

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