Revise set current directory and fill notebooks

[davide-f]

This PR is to revise the importing procedure of all notebooks to make it resilient with respect to the name of the parent directory. All notebooks no more rely on set_to_root function to specify the parent folder

[davide-f]

I'll be merging this PR in the next days unless some comments appear

[ekatef]

@davide-f it feels like a huge relief when no usual path issues appear while going through the notebooks :)

The only small points could be added:

• Paths block in augmented_line_connections seems to need some adjustments;
• in validation_nigeria names of the files could be updated in Definition of main folder paths (like "africa_all_raw_substations.geojson")

[davide-f]

Nice catch! Revised both and tested :)

[davide-f]

Merging :) If little refuses are left (e.g. missing imports of some _helper functions) we may add them later when we discover them