Update SCEN7 writer to handle single variable writing

[znicholls]

Pull request

Please confirm that this pull request has done the following:

• [x] Tests added
• [x] Documentation added (where applicable) (N/A)
• [x] Example added (either to an existing notebook or as a new notebook, where applicable) (N/A)
• [x] 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>`_)

[codecov[bot]]

Codecov Report

Merging #298 into master will increase coverage by 0.00%. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #298   +/-   ##
=======================================
  Coverage   93.37%   93.37%           
=======================================
  Files           7        7           
  Lines        2414     2416    +2     
  Branches      495      495           
=======================================
+ Hits         2254     2256    +2     
  Misses        102      102           
  Partials       58       58           

Impacted Files	Coverage Δ
pymagicc/io.py	93.73% <100.00%> (+<0.01%)	:arrow_up:

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