Add (parallel) netcdf file writer

[apatlpo]

Simple implementation of a parallel netcdf file writer.
Suggestions for improvements are found at #9

[codecov]

Codecov Report

Merging #10 into master will decrease coverage by 1.12%.
The diff coverage is 0%.

@@            Coverage Diff             @@
##           master      #10      +/-   ##
==========================================
- Coverage   83.26%   82.14%   -1.13%     
==========================================
  Files          25       26       +1     
  Lines        4172     4229      +57     
==========================================
  Hits         3474     3474              
- Misses        698      755      +57

Impacted Files	Coverage Δ
shenfun/utilities/nc_writer.py	0% <0%> (ø)

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[mikaem]

Codecov is reminding us that we should add a test for this file as well. Keeping that in mind I merge the PR:-)