Use clearer (example) variable names

[bencomp]

We may be confusing some people when we use eu for a dictionary of countries and their capitals, instead of e.g., capitals_of_eu. I guess this also happens in other notebooks and exercises.

[bencomp]

Another reason for using good variable names: unintentionally reassigning variables causes bugs and confusion. Maybe we should demonstrate how Jupyter keeps variables in memory 'across' cells in the Basics notebook, to show that cell execution order matters.