Compact out readers

[znicholls]

Pull request

Please confirm that this pull request has done the following:

• Hotfix dat writers #281
• Mag read metadata only #288
• Make black #276
• 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>`_)

[znicholls]

@rgieseke and @lewisjared this is good to go I think (following #276). I've made a separate issue for docs (#279) so we don't forget but I don't think there's much point doing docs now as MAGICC7 isn't available.

[lewisjared]

LGTM with some minor refactors

[codecov]

Codecov Report

Merging #277 into master will increase coverage by 0.03%.
The diff coverage is 95.28%.

@@            Coverage Diff             @@
##           master     #277      +/-   ##
==========================================
+ Coverage   94.61%   94.64%   +0.03%     
==========================================
  Files           7        7              
  Lines        2245     2351     +106     
  Branches      449      469      +20     
==========================================
+ Hits         2124     2225     +101     
- Misses         83       87       +4     
- Partials       38       39       +1     

Impacted Files	Coverage Δ
pymagicc/io.py	95.53% <95.28%> (-0.02%)	⬇️

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