vtkDataSetAttributes::FieldList cleanups

The MR does the following:

1. Refactor vtkDataSetAttributes::FieldList. We move it to a separate class vtkDataSetAttributesFieldList (old naming still supported) and rework the implementation to use C++ containers rather than working with raw pointers. We also simply the API and add docs to document how this class is intended to be used.
2. Better support combing named attribute arrays with mismatches. See description for 69a06e4d. This incorporates the changes necessary into FieldList itself.
3. Simplify logic in vtkAppendPolyData and vtkAppendFilter to use vtkDataSetAttributes::FieldList.