Add requested examples to episode 3.4 on functional programming benefits

[bielsnohr]

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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:

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 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)

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