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 would make the doc string something like this:

```
class MovingAverage(object):
  """Helper class for computing moving average."""
```

The `@method` seem redundant when the methods have their own doc strings.