skip meplus fcst prepro by model

### What problem does your feature request solve?
From discussions on https://github.com/MetOffice/CSET/pull/2153, the choice to skip the forecast pre-processing for METPlus must be configurable for each model, currently is hardwired to each site.

### Describe the solution you'd like
A new control variable, configurable in the user interface, for each model, and logic in the app or workflow definitions to skip as required.

### Describe alternatives you've considered
Maintain the forecast pre-processing skipping option at site level, but is not functionally viable.