Generate include files at runtime / remove include files

[jfrost-mo]

What problem does your feature request solve?

Include files are an extra thing to write, maintain and debug, and we should get rid of them. They block adding user provided recipes (#831), and are probably the hardest thing to deal with when contributing a new recipe to CSET.

Describe the solution you'd like

We could automatically generate include files at runtime, based on the variables in the recipes. This might necessitate making a list of standard recipe variables, which wouldn't be a terrible idea, as it could be well documented, and make configuration easier, at the cost of recipe writers needing to be more coordinated.

If we are generating them in one place, we might was well include that in the flow.cylc, and remove include files as a concept entirely.

Describe alternatives you've considered

An extreme alternative would be to remove recipes instead, and just write includes. However this would be a bigger change, and IMO they have a less pleasant syntax.