Add tests of compatibility

[znicholls]

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 <http://link-to-pr.com>`_) Added feature which does something
- (`#XX <http://link-to-pr.com>`_) Fixed bug identified in (`#XX <http://link-to-issue.com>`_)

closes #95 (inadequately but it's a start we can fill out later)

[codecov-io]

Codecov Report

Merging #162 into master will increase coverage by 0.23%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #162      +/-   ##
==========================================
+ Coverage   96.76%   96.99%   +0.23%     
==========================================
  Files           5        5              
  Lines        1142     1165      +23     
  Branches      177      183       +6     
==========================================
+ Hits         1105     1130      +25     
+ Misses         20       19       -1     
+ Partials       17       16       -1

Impacted Files	Coverage Δ
pymagicc/definitions/__init__.py	98.48% <100%> (+0.03%)	⬆️
pymagicc/io.py	97.12% <100%> (+0.33%)	⬆️

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[znicholls]

this should be merged after #163 due to the SCEN renaming done there

[znicholls]

@rgieseke all yours