Update Extreme Rainfall Diagnostics for new models

Currently the extreme rainfall diagnostics are setup for the UM, as that is the data that I currently have available for testing and in a complete format with the meta data to allow sensible use of the diagnostics.

Once other models are available, e.g. LFRic, all recipes will need to be updated for the more generic use across models.

The issue names no files or tests; start by locating the extreme rainfall diagnostic recipes and identifying assumptions tied to UM data and metadata. Done means the recipes support other models such as LFRic without relying on UM-specific structure, once suitable model data is available.

Written by the indexing model from the issue text.