Add AR6 gases

[chrisroadmap]

Pull request

Please confirm that this pull request has done the following:

• [X] Tests added
• [ ] Documentation added (where applicable)
• [ ] Example added (either to an existing notebook or as a new notebook, where applicable)
• [X] Description in CHANGELOG.rst added

Adds in radiative efficiencies etc. from Meinshausen et al., 2017 and Hodnebrog et al., 2013 that were not included in the AR5 gas set.

Getting ready for AR6 - no docs added yet as likely API will change at version 2 - but this keeps things nice and consistent, at least for me, locally

[codecov[bot]]

Codecov Report

Merging #69 into master will increase coverage by 0.11%. The diff coverage is 99.64%.

@@            Coverage Diff             @@
##           master      #69      +/-   ##
==========================================
+ Coverage   97.04%   97.16%   +0.11%     
==========================================
  Files          36       36              
  Lines        1355     1446      +91     
==========================================
+ Hits         1315     1405      +90     
- Misses         40       41       +1

Impacted Files	Coverage Δ
fair/constants/general.py	100% <ø> (ø)	:arrow_up:
fair/constants/br_atoms.py	100% <100%> (ø)	:arrow_up:
fair/constants/cl_atoms.py	100% <100%> (ø)	:arrow_up:
fair/constants/lifetime.py	100% <100%> (ø)	:arrow_up:
fair/constants/molwt.py	100% <100%> (ø)	:arrow_up:
fair/forcing/ghg.py	100% <100%> (ø)	:arrow_up:
fair/constants/radeff.py	100% <100%> (ø)	:arrow_up:
fair/constants/fracrel.py	100% <100%> (ø)	:arrow_up:
fair/constants/preindconc.py	100% <100%> (ø)	:arrow_up:
fair/forward.py	85.56% <93.33%> (+0.32%)	:arrow_up:

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