Ensemble probabilities (subset) for temperature at screen level

### What problem does your feature request solve?

Sometimes when considering ensemble probabilities you may only wish to consider a sub-set of your ensemble, particularly if looking a multi-physics or multi-model ensemble. In these instances being able to create probabilities from a subset is beneficial.

### Describe the solution you'd like

Recipe for ensemble probabilities for a subset of ensemble members.

### Describe alternatives you've considered

The issue names no files, tests, or entry points. Start by locating existing ensemble-probability recipes and the screen-level temperature workflow in CSET, then clarify how callers select ensemble members and what output and validation are expected.

Written by the indexing model from the issue text.