Fixing paths and generating outputs of validation notebooks

[AnasAlgarei]

Adjusted the paths structure to match the recommended path of PyPSA-Earth. Generated outputs of Nigeria for the notebooks for illustration. However , some outputs are not accurate due to issues with the solved network, this will be fixed once a proper solved network is obtained, keeping the notebooks in the PR until then for reference. In addition, minor adjustments have been made for the scripts and markdowns.

[davide-f]

I copy the message from discord to here to keep track. Something like this may be lighter, maybe to be checked and verified.

import os
import sys

if not os.path.isdir("./pypsa-earth"):
    os.chdir("../../..")  # this moves the current directory to the parent of documentation folder, where pypsa-earth folder shall be located as well
    sys.path.append("pypsa-earth/scripts")  # this adds the pypsa-earth/scripts folder into the path

to use the set_paths* function there is already the need to have pypsa-earth located in ../../.., so, it may be very heavy to understand for a new user.

This applies also for #38 and #37