Identify Places to use ._set_index() and Refactor

[chukarsten]

In #3599 , the ._set_index(X,y) function was introduced to the PolynomialDecomposer as a means to use either a pandas.DatetimeIndex or a column in X containing date time information to populate the index of the target (y) with a pandas.DatetimeIndex. It might be desirable to move this function up either to the parent Decomposer class or even higher into the Transformer or Component class for reuse with other functions, particularly other subclasses of Decomposer that are coming down the line.

The work for this story is to identify and enumerate additional issues for other places we might want to use this function, propose a target class for it to migrate to and migrate the function to that class.