automatically calculate `co2base` from selected countries by providing 1990 emission data

[fneum]

Following discussion in #49, we should add a co2base.csv (to the data bundle) giving 1990 electricity sector CO2 emission numbers per country.

If the opts wildcard specifies a relative reduction target, the cap can be automatically calculated based on the countries selected in config["countries"].

We may use values from EDGAR for category 1A1 (power industry) (for 1990, of course): https://edgar.jrc.ec.europa.eu/overview.php?v=432_GHG&SECURE=123

Thinking of pypsa-eur-sec, @nworbmot is there a good per country sectoral co2 emissions data source?