Provide only dask arrays to pyspectral's nir reflectance computation

[mraspaud]

As pyspectral doesn't officially support xr.DataArrays, and with observing that passing DataArrays affects performance badly, this PR changes the call pyspectral's NIR reflectance calculator to dask-only.

• Tests added and test suite added to parent suite
• Tests passed
• Passes flake8 satpy

[coveralls]

Coverage increased (+0.0009%) to 87.204% when pulling 68583a9 on mraspaud:fix-nir-dask into e8f3518 on pytroll:master.

[codecov]

Codecov Report

Merging #1011 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master   #1011      +/-   ##
=========================================
+ Coverage    87.2%   87.2%   +<.01%     
=========================================
  Files         181     181              
  Lines       27890   27892       +2     
=========================================
+ Hits        24321   24323       +2     
  Misses       3569    3569

Impacted Files	Coverage Δ
satpy/composites/__init__.py	78.41% <100%> (+0.05%)	⬆️
satpy/tests/compositor_tests/__init__.py	99.58% <100%> (ø)	⬆️

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[mraspaud]

@djhoese @adybbroe does anyone want to quickly review this ? It's just making sure pure dask arrays are passed to pyspectral, and is linked to pytroll/pyspectral#97

[adybbroe]

Can't see anything alarming or wrong here...