Encode header attributes in CF writer

[sfinkens]

This PR introduces encoding of header attributes in the CF writer. The same methods are employed as for dataset attributes (i.e. the standard json encoder plus some extra rules). This also fixes the problem of zero-valued header attributes not being included in the global nc attributes.

Furthermore, I harmonized the encoding of bool and np.bool_ attributes. Currently we have "True" for np.bool_ and "true" for boolean. I chose "true" and "false" because that is the json default.

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

[coveralls]

Coverage increased (+0.4%) to 86.593% when pulling e4e8584 on sfinkens:fix-cf-writer-header-attrs into 287cffd on pytroll:master.

[codecov]

Codecov Report

Merging #953 into master will increase coverage by 0.4%.
The diff coverage is 95.65%.

@@            Coverage Diff            @@
##           master     #953     +/-   ##
=========================================
+ Coverage   86.19%   86.59%   +0.4%     
=========================================
  Files         174      175      +1     
  Lines       26740    26837     +97     
=========================================
+ Hits        23048    23239    +191     
+ Misses       3692     3598     -94

Impacted Files	Coverage Δ
satpy/writers/cf_writer.py	91.24% <100%> (-0.2%)	⬇️
satpy/tests/writer_tests/test_cf.py	98.37% <92.3%> (-0.2%)	⬇️
satpy/readers/__init__.py	93.07% <0%> (-1.21%)	⬇️
satpy/scene.py	90.35% <0%> (-0.18%)	⬇️
satpy/tests/reader_tests/test_viirs_compact.py	94.33% <0%> (ø)
satpy/tests/reader_tests/__init__.py	98.24% <0%> (+0.03%)	⬆️
satpy/readers/file_handlers.py	97.01% <0%> (+2.98%)	⬆️
satpy/readers/viirs_compact.py	64.56% <0%> (+49.52%)	⬆️

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[mraspaud]

LGTM