Add the CURV diagnostic

### What problem does your feature request solve?

Nigel Roberts has created a diagnostic called CURV (Curvature Using Radial Variation). This diagnostic has many benefits not only in its own right, but also for stratification of diagnostics.

### Describe the solution you'd like

A new operator for this diagnostic and an example recipe. There will be a global model and limited area model implementation.

### Describe alternatives you've considered