Add API methods for setting agent variables by name

[qiemem]

Currently this is a quite complicated process which differs based on whether you're dealing with built-in agent variables, agents-own variables, breeds-own variables, and so forth. We've recently had several extension authors requesting instructions for how to do it. Although it encourage some less than good extension authoring practices (in terms of having extensions depend on variables of specific names and such), we should consider add a setVariable(String, AnyRef) procedure to api.Agent that will work for any variable type.

[SethTisue]

iirc, we historically discouraged this for performance reasons; looking up the index takes time.

regardless, providing the method but documenting any performance downsides seems like a reasonable middle path