Ensemble maximum from a subset

### What problem does your feature request solve?

Produce the ensemble maximum from a subset of the ensemble members.

### Describe the solution you'd like

Recipe.

### Describe alternatives you've considered

No file, test, or entry point is named. Start by locating the existing ensemble maximum recipe and the code that selects ensemble members, then determine how a subset is represented. Done means a recipe can produce the maximum using only the requested subset, with coverage for the intended selection behavior.

Written by the indexing model from the issue text.