Fix cf writing of 3d arrays

[mraspaud]

This is a fix for a bug discovered yesterday by @TAlonglong : the CF writer wasn't able to write images, because of the chunks handling when converting an area to lons and lats.

• Tests added
• Tests passed
• Passes flake8 satpy

[coveralls]

Coverage decreased (-0.009%) to 89.596% when pulling b5bd072 on mraspaud:fix-cf-images into 65cd83f on pytroll:master.

[codecov]

Codecov Report

Merging #1170 into master will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1170   +/-   ##
=======================================
  Coverage   89.60%   89.60%           
=======================================
  Files         200      200           
  Lines       29487    29499   +12     
=======================================
+ Hits        26422    26434   +12     
  Misses       3065     3065           

Impacted Files	Coverage Δ
satpy/tests/writer_tests/test_cf.py	98.57% <100.00%> (+0.03%)	⬆️
satpy/writers/cf_writer.py	93.77% <100.00%> (-0.03%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 65cd83f...b5bd072. Read the comment docs.

[djhoese]

@mraspaud Let me know if this is ready to merge (you can merge it if it is ready). I have a colleague who just ran in to this.

[mraspaud]

@djhoese merged!