Add module interface for the SSPs

For a number of years, the RCP scenarios have been easily importable into FaIR using something like

from fair.RCPs import rcp45
from fair.forward import fair_scm

conc, forc, temp = fair_scm(rcp45.Emissions.emissions)

You can now do the same with the SSP scenarios from CMIP6:

from fair.SSPs import ssp119
from fair.forward import fair_scm

conc, forc, temp = fair_scm(ssp119.Emissions.emissions)

The following are available: ssp119, ssp126, ssp245, ssp370, ssp370_lowNTCF, ssp434, ssp460, ssp534, ssp585

More information is available from O'Neill et al. (2016) and Riahi et al. (2017), and the IPCC's Sixth Assessment Working Group 1 report. The emissions scenarios themselves were compiled by Zeb Nicholls and Jared Lewis and available at https://doi.org/10.5281/zenodo.4589756. Please cite Nicholls et al. (2020) if you use them,

The SSPs are supplied as a convenience as the actual SSP scenarios run for AR6 were done a little differently, but will need a whole configuration change to run as it was there, so it should be noted that results will probably not agree exactly with AR6 or observed history. This is just for people who want to easily get at the scenarios without digging too much into the AR6 repository.