Add new recipe (18) by summer student: correlation calculation

[sadielbartholomew]

The final recipe created by Natalia, which again I have updated a bit to produce only one output plot, of best quality, as per the other recipes with a STDOUT report of the calculation output, plus to tidy and add further comment detail e.g. to describe the data sources. All things Natalia could have done herself had she had more time than the two weeks placement!

Expected output