Interface for conveniently setting up scenarios

A common pattern is to have a small number of discrete scenarios, each with their own sets of variables, and picking one of those to use.

At the moment, you end up defining things like `text1`, `text2`, `...textN`, or for each variable a list containing the choices, then you have to unpack them all with a load of variable definitions of the form `text = texts[scenario]` or `text,parameter,answer = random(zip(texts,parameters,answers)`.

A nice interface for this would be a special kind of variable group called 'scenario definitions', containing a list of scenarios. Adding a variable to the group adds a definition for that variable to each scenario. There's then a field for a JME expression to determine which scenario to use.

No files, tests, or entry points are named. Start by locating the variable-group interface and the handling of JME expressions, then define how scenario definitions add variables to every scenario and select one. Done means users can create scenarios, set per-scenario variables, and choose the active scenario with an expression.

Written by the indexing model from the issue text.