Add better beginner example of function

[cjfullerton]

Instead of kicking off with refactoring a script into a function, begin with a simple example that is built from scratch.

Probably best with something that performs a numerical operation - perhaps converting inflammation units or normalising a data set?

If we start with a function that performs a simple numerical operation, we can demonstrate that functions operate on arrays for free.

[cjfullerton]

Content essentially done for this. There's a bit of repetition - read through and remove this.