Calculate co2base depending on selected countries

Describe the feature you'd like to see

Currently, the co2 emissions for the 1990 base year are hard-coded in the config for Europe only. When the user wants to optimize only one country or a set of countries, they have to manually check and change the emissions for 1990.
https://github.com/PyPSA/pypsa-eur/blob/5bf439009602ee950e509e12d1a4664ffc7bae54/config.default.yaml#L35-L40

What is proposed here, is to calculate the co2base automatically.
A simple string could activate this automatic co2base calculation:

co2base: "automatic"  # "automatic" will set the co2base according to XXX, use any float number to define the co2base manually 

A function which retrieves the data for the given countries and that calculates the co2base could be implemented in a "utils" folder and imported where the co2 constraints are defined. Depending on the data situation, we could host a CSV with all emissions per sector for all countries in the world. In case no data is available for a given country, an assertion is triggered which invites users to contribute detailed assumptions with references.

Start by inspecting config.default.yaml at lines 35-40 and the existing co2 constraint definition. Determine how the selected countries are represented and where emissions data could be sourced or stored. Done means an automatic co2base is calculated for the selected countries, manual float values still work, and missing country data triggers an assertion with an invitation to contribute assumptions and references.