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 `DataSet`s 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.
fixed some typo
more Dosygen updates, remove necessary #include
clarify on name lookup for overloaded virtural function
minor refine on the name lookup rule
rename subdirectory
move virtual functions to .cxx, apply NVI pattern
fixed EXPORT macro
make namespace reflect directory structure
