- reduces code, simplifies creation of new, specialized polyData
  writers.

- new templated vtk::GenericPatchWriter, which adds support for
  writing both uindirectPrimitivePatch + indirectPrimitivePatch types.

- handle geometric fields separately from regular fields with
    * vtk:GenericPatchGeoFieldsWriter
    * vtk:indirectPatchGeoFieldsWriter
    * vtk:uindirectPatchGeoFieldsWriter