Add requested examples to episode 3.4 on functional programming benefits

Along the line, @anenadic requested further details around how functional programming can be helpful but left those comments in the course material itself. This is towards the end of [Testing Pure Functions](https://carpentries-incubator.github.io/python-intermediate-development/34-code-refactoring.html#testing-pure-functions):

> There are no definite rules in software design but making your complex logic out of composed pure functions is a great place to start when trying to make your code readable, testable and maintainable. This is particularly useful for:
>
> - Data processing and analysis (for example, using [Python Pandas library][1] for data manipulation where most of functions appear pure)
> - Doing simulations (? needs more explanation)
> - Translating data from one format to another (? an example would be good)
>
> [1]: https://pandas.pydata.org/

We should add the examples/explanation requested in the text.