Fix nir reflectance to use XArray

[mraspaud]

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

[coveralls]

Coverage increased (+0.02%) to 67.3% when pulling d7bc3e6 on fix-nir-reflectance into 3528bc5 on master.

[codecov]

Codecov Report

Merging #318 into master will increase coverage by 0.02%.
The diff coverage is 25%.

@@            Coverage Diff            @@
##           master    #318      +/-   ##
=========================================
+ Coverage   67.27%   67.3%   +0.02%     
=========================================
  Files         120     120              
  Lines       14675   14676       +1     
=========================================
+ Hits         9873    9877       +4     
+ Misses       4802    4799       -3

Impacted Files	Coverage Δ
satpy/composites/__init__.py	32.88% <0%> (-0.05%)	⬇️
satpy/readers/hrit_msg.py	21.46% <100%> (ø)	⬆️
satpy/readers/__init__.py	94.96% <0%> (+1.55%)	⬆️

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