Add configuration file to drop dependency of world_iso and build_demand

[davide-f]

Describe the feature you'd like to see

After #814 is solved, an interesting feature is to decouple the overloading of world_iso and its use in build_demand to characterize which nc file to load by country.
I personally believe that it is safer to decouple the two dependencies:

• have 1 dictionary to clarify what countries are covered by each GEGIS nc file
• have 1 dictionary to store the country classification that we desire

Currently we are adopting a unique dictionary for both and that hinders the flexibility of defining the regional scope of the different entries

[Tomkourou]

Please assign me.

[davide-f]

Closed by #822