Fix DayNightCompositor not checking inputs areas

[pnuu]

This PR fixes an oversight in DayNightCompositor that results in zero-sized composite if the areas of the two unresampled composites do not match.

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

[coveralls]

Coverage increased (+0.02%) to 69.053% when pulling 2e45288 on bugfix-DNC-different-shapes into b8afac5 on master.

[codecov]

Codecov Report

Merging #395 into master will decrease coverage by <.01%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master     #395      +/-   ##
==========================================
- Coverage   69.05%   69.05%   -0.01%     
==========================================
  Files         123      123              
  Lines       15739    15740       +1     
==========================================
  Hits        10869    10869              
- Misses       4870     4871       +1

Impacted Files	Coverage Δ
satpy/composites/__init__.py	32.78% <0%> (-0.05%)	⬇️

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[djhoese]

All composites should have a check_areas method. Could you switch to using that instead.

[pnuu]

Oh. One learns something new every day. I'll update later today.