Plumbing to connect aggregated scores metrics into a Hinton plot

### What problem does your feature request solve?

We have an operator that produces a Hinton plot, but we need an operator that can pull in multiple metrics into a scorecard and arrange this in a sensible way. This might be for aggregated statistics (this would be covered separately).

We would want to be able to cater for CRPS, RMSE e.g. when we have two models that have been compared against obs, to show the relevant change in skill score. We would also want to expand this to handle FSS statistics, where the y axis is the rain threshold.

If two models, then we only need to produce one set of plots.
If more than two models, we would have to work out the permutations of these and plot each possible model comparison scorecard.

### Describe the solution you'd like

### Describe alternatives you've considered

Start by locating the existing Hinton plot operator and the operators that produce or aggregate CRPS, RMSE, and FSS metrics. Define how scorecards should compare two or more models, including FSS rain-threshold axes and model permutations. Done means the supported metrics can be connected to Hinton plots with sensible output for the stated model cases.

Written by the indexing model from the issue text.