Warn and/or remove column when split_by is used and no values are present for a level of the splitting variable

[tyarkoni]

Suppose we pass a variable to add_term() and set split_by='condition'. If the target variable has no valid values for one or more levels of the splitting variable, a constant term will be added to the model. This is typically not desirable. We should check for this and, at minimum, issue a warning. We should probably also implement a general check to make there is (at most) a single constant column in the design.