cf_writer supports array type in attrs

[zxdawn]

If the attribute type is array, the cf_writer could cause the error which is similar to #1243 .

• Tests added
• Tests passed
• Passes flake8 satpy

[coveralls]

Coverage decreased (-0.003%) to 90.024% when pulling 1088d8f on zxdawn:cf_writer into 9261a1c on pytroll:master.

[codecov]

Codecov Report

Merging #1245 into master will decrease coverage by 0.00%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master    #1245      +/-   ##
==========================================
- Coverage   90.02%   90.02%   -0.01%     
==========================================
  Files         218      218              
  Lines       31263    31265       +2     
==========================================
+ Hits        28145    28146       +1     
- Misses       3118     3119       +1     

Impacted Files	Coverage Δ
satpy/writers/cf_writer.py	93.47% <50.00%> (-0.31%)	⬇️

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