Cell effective radius plots

### What problem does your feature request solve?
Create cell effective radius plots which, for every cell, calculates the radius of a circle with the same area as the cell for plotting.
RES has thresholds for these at 0.5, 1, 2, 4, 8, 16, 32, 64, 128 mm (for precip).

### Describe the solution you'd like
A provisional plan:

- Identify region
- Read model field
- Mask to region (if needed)
- Remove missing data
- Calculate effective radius of cells
- Plot

Observations should be an option for the plots once available, see #403.
_Note_: In the RES this is currently only done for precipitation.

### Describe alternatives you've considered

Start by tracing the existing region, model-field, masking, missing-data, and plotting workflow; no specific files or tests are named. Define the precipitation effective-radius calculation against the RES thresholds from 0.5 to 128 mm and confirm that cell plots work, with observations as an option once #403 is available.

Written by the indexing model from the issue text.