Improvements on Zambia validation notebook

[asolavi]

In this issue, improvements to the Zambia validation notebook are proposed:

1. Specify that the pypsa-earth model with Zambia as country needs to have been executed. To do so:
   a. Add a specific config file in the configs folder that runs the "default" version of pypsa-earth in Zambia.
   b. At the beginning of the notebook, write a paragraph specifying that pypsa-earth for Zambia needs to have ran before doing the validation, and link to the config file added in a. above.

2. Specify where the notebook Zambia_validation.ipynb needs to be placed so that the path definitions work for everyone. Ideally, they should work for the following "standard" configuration:
   There is a folder that contains the pypsa-earth and pypsa-zm-data repository folders in them. The name of this "container" folder can be whatever the user chooses, so the paths need to be defined accordingly.
   Ex: If the "container" folder is called repos, then the repositories are in ..\repos\pypsa-earth\ and ..\repos\pypsa-zm-data\.

3. Remove the definition of functions in the notebook, by obtaining them from the environment. (I'm aware this can be tricky, but let's look into it). The point 2. above should be helpful for this.