If not a Series it should refer to a column in the choosers table.
We're usually passing in a column from the choosers table anyway,
so it makes sense to allow it to be specified this way.

This is somewhere between a RegressionModel and the
RegressionModelGroup. Unlike the group the individual models are not
fully configurable: only the model expression and ytransform can be
customized. Fit and predict filters are global across all segments.

YAML interface is still to be done.

This appears to be working with my workflow where I edit the yaml directly.  The biggest issue was when using numpy types as the segmented keys (when building_type_ids are ints).  Also edited the yamlmodelrunner to peek into the config file and choose the segmented or non-segmented model based on the model_type.  I will definitely be making these segmented models most of the time so I'm not sure if the forking strategy is really necessary.