Ensemble spread about the mean (including control)

### What problem does your feature request solve?

This takes the standard deviation of the ensemble, and is thus considered the spread about the mean.

### Describe the solution you'd like

Recipe.

### Describe alternatives you've considered

The issue names no files, tests, or entry points; start by locating existing ensemble recipes in CSET and their tests. The work is complete when a recipe calculates ensemble spread as the standard deviation about the mean, including the control member, with coverage for the intended behavior.

Written by the indexing model from the issue text.