Add a rule to save data in a format compatible with the IPCC AR6 database

[martavp]

The idea is to add a rule 'fill_IPCC_template' (for the myopic approach and after the rule 'make_summary') which saves the results using the template of the 6th Assessment Report database.

We could reuse some of the work in: https://github.com/martavp/pypsa-eur-sec-to-ipcc/

This is not urgent since the call to report scenarios for the 6th Assessment Report closed today, but it would be great to have this included for future IPCC reports.

[fneum]

Could also combine with output in IAMC time series format https://pyam-iamc.readthedocs.io/en/stable/data.html

[martavp]

We have a working version that "translates" pypsa-eur-sec v0.5.0 to IMAC format https://github.com/martavp/pypsa-eur-sec-to-ipcc/blob/main/pypsa-to-iamc We plan to include this as a rule and create a pullrequest sometime soon :)