Feature/backend kwargs json to netcdf

[EddyCMWF]

Allow backend kwargs to be provided in the to_netcdf executable, either via a json format string, or a path to a json file.
Fixes #286

[EddyCMWF]

Hi @alexamici , can you review this PR which fixes issue #286

[codecov-commenter]

Codecov Report

Merging #288 (0abd67d) into master (8e170ec) will decrease coverage by 0.28%.
The diff coverage is 53.33%.

@@            Coverage Diff             @@
##           master     #288      +/-   ##
==========================================
- Coverage   95.30%   95.01%   -0.29%     
==========================================
  Files          26       26              
  Lines        1938     1947       +9     
  Branches      229      230       +1     
==========================================
+ Hits         1847     1850       +3     
- Misses         61       66       +5     
- Partials       30       31       +1     

Impacted Files	Coverage Δ
cfgrib/__main__.py	83.05% <53.33%> (-8.95%)	⬇️

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[alexamici]

@EddyCMWF very nice feature!

I have a few style comments:

• you need to format the code with black you may run the make command to fix code style
• why do you move imports inside the code? this is a but confusing and both modules are non-optional

[EddyCMWF]

My thinking was to improve performance by only importing modules if they are used, but happy to put the imports back to the top of the function.

(Side note, black made my code non-PEP8 compliant _O_/)

[alexamici]

@EddyCMWF

My thinking was to improve performance by only importing modules if they are used, but happy to put the imports back to the top of the function.

My rule is: "readability over performance, unless there is a benchmark" 😄

[EddyCMWF]

@alexamici , sorry to change after your approval, I realised this should have a test in place, so have just added one

[iainrussell]

Hi @EddyCMWF, thanks for adding the tests! I'm happy to merge this into master if you're finished now.

[EddyCMWF]

Hi @iainrussell , yes I am finished now