Update latest model adjustment code and separate all code to scripts/functions

[UmerYasin1]

This PR extends the MESSAGEix Pakistan model with a new adjustment for the baseline scenario. Put code in different scripts and functions and call them into our main model file. In this script, we adjust the model by adding new demands, renewable capacities, new historical activity data, and percentage of shares for renewable electricity sources and adjust electricity generation values by putting different constraints.

• Update the main model file and add some content to explain what exactly happens in that script.
• Created file_strucutre.md file to walk through all the files used in the model.
• Add Model_progress.md file to review overall activity done to build a baseline model.

How to Review

• Check out this branch, run the model (pakistan_model.ipynb), and ensure it works without a flaw (error or infeasibility).
• Read the comments and doc written in model files.
• Review the script written in a separate folder.

PR checklist

• The model run without any error.
• Give new electricity generation profile