Add more robust test of io

[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) (N/A, all internal)
• 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>`_)

[znicholls]

@lewisjared can you take a look when you've got a minute? Let's not merge until @rgieseke gives the approval though

[lewisjared]

Are any of these expected files actually run through magicc to be sure that they work?

[znicholls]

Are any of these expected files actually run through magicc to be sure that they work?

They are now in tests/test_core.py::test_pymagicc_writing_has_an_effect (except the OT file because I have no idea how that's used and hence how to test it...)

[znicholls]

@rgieseke if you have time would be great to get your thoughts

[rgieseke]

Excellent documentation of pit-falls, very useful to have this written down here!
Only question from my side the debug statement ...

[znicholls]

@lewisjared over to you to merge when you're happy then I think