Aggregation in Q-Q plots

### What problem does your feature request solve?

Currently the Q-Q plots can only be calculated for a full forecast. They cannot be calculated over multiple forecasts (i.e. aggregated).

### Describe the solution you'd like

It would be sensible to aggregate over multiple forecasts, particularly via lead time and all points (as in histograms). I am unsure if we would want via validity time or diurnal cycle, although the latter could be interesting at times and for certain variables.

### Describe alternatives you've considered

None.

Start by locating the Q-Q plot implementation and the existing histogram aggregation behavior. Determine how aggregation by lead time and all points should work, and clarify whether validity time or diurnal-cycle aggregation is required; done should include agreed aggregation options with coverage for the supported cases.

Written by the indexing model from the issue text.