Add netCDF support V2

[lewisjared]

Pull request

Please confirm that this pull request has done the following:

• Tests added
• Documentation added (where applicable)
• Example added (either to an existing notebook or as a new notebook, where applicable)
• Description in CHANGELOG.rst added

Adding to CHANGELOG.rst

Please add a single line in the changelog notes similar to one of the following:

- (`#XX <https://github.com/openscm/scmdata/pull/XX>`_) Added feature which does something
- (`#XX <https://github.com/openscm/scmdata/pull/XX>`_) Fixed bug identified in (`#YY <https://github.com/openscm/scmdata/issues/YY>`_)

Requires:

• Split ScmDataFrame into ScmRun and ScmEnsemble #21

Supercedes #26

Closes #15

[codecov-io]

Codecov Report

Merging #34 into master will decrease coverage by 2.51%.
The diff coverage is 72.00%.

@@            Coverage Diff             @@
##           master      #34      +/-   ##
==========================================
- Coverage   94.29%   91.77%   -2.52%     
==========================================
  Files          11       12       +1     
  Lines        1315     1484     +169     
  Branches      281      318      +37     
==========================================
+ Hits         1240     1362     +122     
- Misses         50       89      +39     
- Partials       25       33       +8     

Impacted Files	Coverage Δ
src/scmdata/netcdf.py	69.23% <69.23%> (ø)
src/scmdata/run.py	92.64% <75.00%> (+0.03%)	⬆️
src/scmdata/__init__.py	100.00% <100.00%> (ø)
src/scmdata/dataframe.py	96.53% <100.00%> (+0.07%)	⬆️
src/scmdata/testing.py	100.00% <100.00%> (ø)
src/scmdata/groupby.py	86.20% <0.00%> (+1.72%)	⬆️

---

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 804d9ee...34737f2. Read the comment docs.

[znicholls]

If we want other reviewers, we could always ping IAMconsortium/pyam#335

[znicholls]

Huge @lewisjared this looks great!