Implement ForecastingDetectorFactory

[williewheeler]

Currently we use various legacy factories to create ForecastingDetectors from a specific point forecaster. All of the legacy factories use the Welford interval forecaster.

We need a ForecastingDetectorFactory for two reasons:

• it will allow us to create detectors for the naive forecaster and seasonal naive forecaster without adding two more legacy detectors to the mix, and
• it will allow us to remove the legacy factories

Need to ensure that we migrate any detectors using the old formats though.