Cell mean value plots

### What problem does your feature request solve?
Create cell mean value plots which take the mean of values within all cells over a domain.
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 cell centre
- Compute the mean of all 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

No files, tests, or entry points are named. Start by locating the existing region and model-field plotting workflow, then trace how masking, missing values, cell centres, and aggregation are handled. Done means a cell-mean plot supports the described workflow, precipitation thresholds, and an observations option coordinated with #403.

Written by the indexing model from the issue text.