Revised training (Training-update branch) - Update sections 5, 6 and 7 (old sections 7, 8 and 14)

[aidanmews]

Include:

• Discuss and/or example A well named sub-function is better than a comment.
• Discuss and/or example Many small functions > one big function.
• Add functions to R/ (probably 2 functions that are designed to allow us to demonstrate thorough unit testing – one doing something with data – might also help to demonstrate dplyr being used programmatically/ in a package.) (replaces 7:8)