Add required pip install commands at the top of modules

[pansapiens]

This will make each module self contained and ensure dependencies are there no matter which Jupyter-style notebook people use.

eg

!pip install pandas numpy plotnine

[pansapiens]

Fixed in indexing module.

[pansapiens]

• [x] intro
• [x] working with data
• [x] indexing
• [x] loops
• [x] plotting with plotnine (ggplot)