SegmentedRegressionModel

This adds a `SegmentedRegressionModel` class that falls in between a single `RegressionModel` and a `RegressionModelGroup` in terms of configurability. As with `RegressionModelGroup` you can specify models individually per segment, but filters apply globally across all segments and are in fact applied before doing and fit or prediction on the segments.

The `SegmentedRegressionModel` may optionally be configured with a default model and ytransform that can be overridden for individual segments.