Extreme Rainfall Diagnostics

### What problem does your feature request solve?

It is really useful to understand the representation of extreme events through physical processes. Recent research from Davies et al. ([2024](https://doi.org/10.1016/j.wace.2024.100696), [2026](https://doi.org/10.1016/j.wace.2025.100846)) developed a four-stage conceptual model for life-threatening sub-hourly rainfall extremes. The conceptual model is built upon a number of diagnostics and properties of said diagnostics. This super issue aims to implement these extreme rainfall diagnostics into CSET.

The operators and diagnostics in this ticket are strongly linked to #1852.

### Describe the solution you'd like

A series of diagnostics that enable a physical examination of extreme rainfall.

### Describe alternatives you've considered

One option is to code these directly in the model. However, it is useful to be able to calculate them offline from past model data too without re-running the model. It will also allow calculation from other models based on their output.