Ensemble mean (subset)

### What problem does your feature request solve?

Ensemble mean for a subset of the ensemble. Applicability needs to be clear in the documentation as it is not always appropriate to use the ensemble mean.

### Describe the solution you'd like

Recipe.

### Describe alternatives you've considered

No file, test, or entry point is named. Start by clarifying the requested recipe and how selecting a subset should work, then document when an ensemble mean is appropriate and what completion criteria the recipe must meet.

Written by the indexing model from the issue text.