Custom effect measures

[jmafoster1]

The causal test engine is currently very limiting WRT causal effect measures. It only supports ate, cate or risk ratio but there are others (that we have even implemented)! The reason for this is that the mapping from JSON string to method execution seems to be hardcoded in an if statement in the test engine. I think we need to generalise this a bit so users can implement custom effect modifiers (or use all the ones we already have) without having to modify the code of causal test engine.