Time axis serialisation

[znicholls]

Pull request

Please confirm that this pull request has done the following:

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

Closes #141

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>`_)

[codecov]

Codecov Report

Merging #143 (136d60e) into master (a73a239) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #143      +/-   ##
==========================================
+ Coverage   93.78%   93.83%   +0.04%     
==========================================
  Files          15       15              
  Lines        1610     1622      +12     
  Branches      326      327       +1     
==========================================
+ Hits         1510     1522      +12     
  Misses         79       79              
  Partials       21       21              

Impacted Files	Coverage Δ
src/scmdata/netcdf.py	95.91% <100.00%> (+0.26%)	⬆️

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[znicholls]

Part of the solution was in #129