Configure UM via models

[penguian]

Make the UM Spack package more user configurable by adding variants corresponding to the variables in packages/um/rose-app.conf.

Create a model subdirectory and a model variant, and move the existing packages/um/rose-app.conf to packages/um/model/access-ram3/rose-app.conf. The model variant is to be used to obtain default values for the other variants.

The exception is the UM version itself, which must be configured as usual, with a check that the UM version and the model are compatible.