Anomaly call likelihood

pass correct values to `anomaly_likelihood`, enable `Anomaly.MODE_LIKELIHOOD` 
and provide test (serving as an example how to use the Anomaly class)

Fixes #967 
I believe the canonical usage is so always use the instance reference even if you're calling a static or class method. That way you don't have to update if you change the name of the class. And I think it works better with inheritance.