Clustering countries to reduce number of countries to be solved

[gincrement]

Within the current implementation there is the need to have at least 1 node for each country: N_countries <= N_nodes

In some studies, it would be helpful to combine e.g., the countries of former Yugoslavia (Slovenia, Croatia, Bosnia and Herzegovina, Serbia, Montenegro, Kosovo, and North Macedonia) into one node as their power demand is quite small and not part of the study focus.

As such, the demands as well as the generation and storage technologies need to be summed up and the transmission within the group of countries can be ignored as it is done with large countries if only 1 node is chosen for them.

Does anyone else see this useful?

[fneum]

Yes, useful, but it's not going to be an easy change as much of the downstream logic works with subdividing country-level statistics.