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 
It seems dangerous to cast as an integer here. What if it isn't an integer, what will happen? We don't know so it would be better to check that it is an int at the beginning of the function and throw a TypeError if it isn't. Then we don't need the cast here.