Support user provided recipes in workflow

[jfrost-mo]

What problem does your feature request solve?

Currently the workflow requires that new recipes be added to the CSET python package, then also added to the workflow via an include file. This is quite a lot of work, and prevents users adding new recipes without being reasonably experienced developers, hampering the flexibility of CSET.

Describe the solution you'd like

We should allow users to add recipes as simple file paths. a list input in rose edit would be a good way of allowing however many recipes are needed to be specified.

Describe alternatives you've considered

To do this properly we really need to get rid of include files, and have them automatically generated at runtime. Alternatively we could only support recipe files without any variables, and just use a static include file for them all.

We may also run into issues if the recipes titles clash, as then they will use the same underlying folder, so this may require resolving.