Forcing reference

[chrisroadmap]

Pull request

Please confirm that this pull request has done the following:

• [x] Tests added
• [x] Description in CHANGELOG.rst added

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and no project coverage change.

Comparison is base (7ebd484) 94.93% compared to head (77edaa3) 94.94%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #123 +/- ## ======================================= Coverage 94.93% 94.94% ======================================= Files 19 19 Lines 731 732 +1 Branches 104 105 +1 ======================================= + Hits 694 695 +1 Misses 16 16 Partials 21 21 ``` | [Impacted Files](https://app.codecov.io/gh/OMS-NetZero/FAIR/pull/123?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OMS-NetZero) | Coverage Δ | | |---|---|---| | [src/fair/fair.py](https://app.codecov.io/gh/OMS-NetZero/FAIR/pull/123?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=OMS-NetZero#diff-c3JjL2ZhaXIvZmFpci5weQ==) | `91.10% <100.00%> (+0.02%)` | :arrow_up: |

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Do you have feedback about the report comment? Let us know in this issue.

[chrisroadmap]

Allows a user to set a different baseline concentration in meinshausen2020 to the baseline_concentration used in the run. This would allow methane and nitrous oxide to be run with a natural emissions source with a baseline concentration of zero and pre-industrial concentration used in the forcing calculation.