Fix attribute datatypes in CF Writer

[sfinkens]

The CF writer currently converts all numpy integer/float attributes to Python int/float. This was originally added to facilitate serialization of complex attributes (such as raw_metadata) using json. But since it is also applied to other attributes, it causes mismatches between the type of an nc variable and its _FillValue attribute for example.

This PR preserves the datatype of attributes, if possible, and adds a separate method to support json serialization.

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

[mraspaud]

LGTM, will merge when the tests pass

[codecov]

Codecov Report

Merging #867 into master will decrease coverage by <.01%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #867      +/-   ##
==========================================
- Coverage   84.62%   84.62%   -0.01%     
==========================================
  Files         169      169              
  Lines       24988    24996       +8     
==========================================
+ Hits        21147    21152       +5     
- Misses       3841     3844       +3

Impacted Files	Coverage Δ
satpy/tests/writer_tests/test_cf.py	98.54% <100%> (+0.01%)	⬆️
satpy/writers/cf_writer.py	91.43% <85%> (-0.55%)	⬇️
satpy/readers/avhrr_l1b_gaclac.py	90.14% <0%> (-1.41%)	⬇️

---

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 4543926...7c4ee7a. Read the comment docs.

[coveralls]

Coverage decreased (-0.003%) to 84.622% when pulling 7c4ee7a on sfinkens:fix-cf-writer-dtype into 4543926 on pytroll:master.