Taking carbon cycle functionality out of concentrations_emissions_han…

[maritsandstad]

…dler and into separate module carbon_cycle_mod

Pulling out carbon cycle stuff in separate module and trying to add carbon cycle output. Primarily airborne fraction of CO2, but possibly also accounting for the different carbon pools.

• [x] Tests added
• [x] Documentation added
• [ ] Example added (in the documentation, to an existing notebook, or in a new notebook)
• [x] Description in CHANGELOG.rst added (single line such as: (`#XX <https://github.com/ciceroOslo/ciceroscm/pull/XX>`_) Added feature which does something)

[maritsandstad]

TODO:

• Tests for constant total carbon.
• Cumulative -> fluxes
• Get ocean pool in concentration run case