Add netcdf support

[lewisjared]

This will likely be heavily modified when we transition to the new data model, but reading writing nc files might be quicker in the mean time.

Also it allows writing output without the pyam dependency from to_csv

[lewisjared]

Still need to add a reader

[lewisjared]

Ignore the ready for review at the moment. The button moved as I clicked...

[codecov]

Codecov Report

Merging #26 into master will decrease coverage by 1.55%.
The diff coverage is 87.6%.

@@            Coverage Diff             @@
##           master      #26      +/-   ##
==========================================
- Coverage   98.82%   97.27%   -1.56%     
==========================================
  Files           7        8       +1     
  Lines         767      881     +114     
  Branches      152      177      +25     
==========================================
+ Hits          758      857      +99     
- Misses          9       19      +10     
- Partials        0        5       +5

Impacted Files	Coverage Δ
src/scmdata/dataframe.py	99.75% <100%> (ø)	⬆️
src/scmdata/__init__.py	100% <100%> (ø)	⬆️
src/scmdata/rw.py	85.43% <85.43%> (ø)

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[znicholls]

One very quick thought, iris has to/from dataframe methods and will do all the dependency handling for us so might be an easier way than using netCDF4 directly?