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

[sadielbartholomew]

Given how busy Ankit is with his PhD we have agreed I can merge this without review. At release time I will enact the plan I outlined above:

let me suggest that I add all five recipes via PRs like this one, with the Python script and addition to the recipe_list.txt, then I do a final PR where I add all of the built material from those five to the repo finally, show they show up on the site

So, merging, and there will eventually be a follow-on PR to get all the new recipes built and included.