Ensemble mean (not including control)

### What problem does your feature request solve?

The ensemble mean for the perturbed members. Again ensure documentation includes full problems with the ensemble mean at convective-scale as it is not (always) an appropriate measure.

### Describe the solution you'd like

Recipe.

### Describe alternatives you've considered

Start by locating existing recipe implementations and the documentation covering ensemble means. Clarify how perturbed members should be selected while excluding the control, and identify the project’s expected output and validation approach. Done means a usable recipe plus documentation of the convective-scale limitations of the ensemble mean.

Written by the indexing model from the issue text.