Add new recipe (19) as started by summer student: per-season trends

[sadielbartholomew]

A recipe I constructed based on the beginnings of a recipe produced by our second summer student, George. I decided it would be good to go on from his basis to show how simple it is for cf-python to calculate seasonal trends and to plot them with cf-plot, which is a bit different to anything shown in the existing recipes, as far as I have seen.

Expected output