Ensemble mean (including control)

### What problem does your feature request solve?

Ensemble mean including the control member. Note the documentation should be clear that on convective-scales the ensemble mean is not always (and generally not) an appropriate diagnostic to consider.

### 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 recipe implementations and their documentation, then determine how ensemble members and the control member are represented; done means a recipe calculates the ensemble mean including control and clearly documents the limitation for convective-scale diagnostics.

Written by the indexing model from the issue text.