Create mapping for gegis demand data

[davide-f]

Describe the feature you'd like to see

Improve the flexibility of build_demand to specify the continent file by country.
Note that in our approach, for example, russia is in Asia, but the demand profile is stored in the Europe file.
The recommended revision is to create a dictionary in the file configs/osm_config.yaml that stores that mapping. It may be food also to revise the name of the file "osm_config" into something like "regions_config.yaml"