Update VIIRS/MODIS ReflecanceCorrector modifier to download DEM data at runtime

[djhoese]

This modifier is the last bit of Satpy that I know of that uses an external file for additional functionality. With this PR it now downloads the files specified by the YAML so the results are as accurate as possible.

• Tests added
• Fully documented
• Move modifier to satpy.modifiers subpackage
• Refactor tests to be cleaner and less duplicated code

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[codecov]

Codecov Report

Merging #1618 (be132b8) into master (3623c03) will increase coverage by 0.21%.
The diff coverage is 99.34%.

@@            Coverage Diff             @@
##           master    #1618      +/-   ##
==========================================
+ Coverage   92.38%   92.60%   +0.21%     
==========================================
  Files         254      258       +4     
  Lines       37434    37716     +282     
==========================================
+ Hits        34584    34927     +343     
+ Misses       2850     2789      -61     

Flag	Coverage Δ
behaviourtests	4.81% <7.54%> (+0.03%)	⬆️
unittests	92.74% <99.34%> (+0.21%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
satpy/composites/viirs.py	86.48% <ø> (-0.06%)	⬇️
satpy/tests/compositor_tests/test_viirs.py	100.00% <ø> (+0.31%)	⬆️
satpy/modifiers/_crefl.py	98.88% <98.88%> (ø)
satpy/tests/modifier_tests/test_crefl.py	99.53% <99.53%> (ø)
satpy/modifiers/atmosphere.py	50.54% <100.00%> (+0.54%)	⬆️
satpy/tests/modifier_tests/__init__.py	100.00% <100.00%> (ø)
satpy/readers/seviri_l1b_hrit.py	90.18% <0.00%> (-0.79%)	⬇️
satpy/dependency_tree.py	97.34% <0.00%> (-0.66%)	⬇️
satpy/scene.py	92.17% <0.00%> (-0.32%)	⬇️
satpy/readers/mirs.py	71.65% <0.00%> (-0.29%)	⬇️
... and 27 more

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[mraspaud]

LGTM

[mraspaud]

Anything you can do for the codebeat comment (for crefl that is)?

[djhoese]

I'll take a look. FYI we're doing some testing with Polar2Grid that uses this algorithm and I think I need to un-daskify it. Perhaps wrap it in a map_blocks function. I think it is one reason we are seeing major performance issues in my new version of P2G (performance updates in a later PR of course).

[djhoese]

Codebeat issues related to crefl have been fixed. The code is so much better.

Since you approved I will expect either a change of review from you or I will merge it when it passes the tests.

[mraspaud]

Yep, go ahead.