xarray.to_netcdf kwargs and tests

[EddyCMWF]

Functionality to allow providing kwargs to xarray.to_netcdf in the cfgrib to_netcdf executable.
For example to use alternate engines or additional compression options.
The new calling syntax is:
cfgrib to_netcdf INFILE OUTFILE --netcdf_kwargs_json TO_XARRAY_KWARGS_JSON

[codecov-commenter]

Codecov Report

Merging #294 (409c945) into master (125b1df) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #294      +/-   ##
==========================================
+ Coverage   95.35%   95.39%   +0.04%     
==========================================
  Files          26       26              
  Lines        1957     1978      +21     
  Branches      230      231       +1     
==========================================
+ Hits         1866     1887      +21     
  Misses         61       61              
  Partials       30       30              

Impacted Files	Coverage Δ
cfgrib/__main__.py	93.93% <100.00%> (+0.71%)	⬆️
tests/test_60_main_commands.py	100.00% <100.00%> (ø)

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