Ensemble spread of a subset about the mean

### What problem does your feature request solve?

The ensemble spread of a subset about that subsets mean.

### Describe the solution you'd like

Recipes

### Describe alternatives you've considered

Start by reviewing the repository's existing recipes and how they describe ensemble statistics. Clarify the expected subset inputs, mean-based spread calculation, and output before identifying the appropriate recipe location; done means the requested subset spread is documented and covered by an example or validation.

Written by the indexing model from the issue text.