Add `CreateResult` to `NewFilter` and absorb field mapping

The original version of Filter classes had a helper header file named CreateResult.h that had several forms of a CreateResult function that helped correctly create the DataSet to be returned from a filter's DoExecute. With the move to the NewFilter structure, these functions did not line up very well with how DataSets should actually be created.

A replacement for these functions have been added as protected helper methods to NewFilter and NewFilterField. In addition to moving them into the filter themselves, the behavior of CreateResult has been merged with the map field to output functionality. The original implementation of Filter did this mapping internally in a different step. The first design of NewFilter required the filter implementer to call a MapFieldsOntoOutput themselves. This new implementation wraps the functionality of CreateResult and MapFieldsOntoOutput together so that the DataSet will be created correctly with a single call to CreateResult. This makes it easier to correctly create the output.