Update naming conventions

[znicholls]

Pull request

Another step on the slow walk towards consistent RCMIP naming (this just hot-fixes a few variables)

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) (N/A)
• 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]

Codecov Report

Merging #317 (a520731) into master (07a7e9f) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #317      +/-   ##
==========================================
+ Coverage   93.29%   93.31%   +0.01%     
==========================================
  Files          18       18              
  Lines        2490     2497       +7     
  Branches      486      488       +2     
==========================================
+ Hits         2323     2330       +7     
  Misses        105      105              
  Partials       62       62              

Impacted Files	Coverage Δ
pymagicc/definitions/__init__.py	98.02% <100.00%> (+0.07%)	⬆️

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[znicholls]

@lewisjared fyi this unifies things a bit more (not quite RCMIP compatible yet but getting there)