Some level of support was previously available for coloring by field
data, but this was restricted to drawing triangle strips. This patch
adds another mode to coloring by field data that enables coloring the
entire data set by a particular tuple in the field data. The field
data tuple gets mapped through the color map, and that color is used
to color the entire data set. The default behavior for triangle strips
is retained by setting this tuple index to -1, the default value.

The motivation for this change is to make it possible to color blocks
in a multi-block dataset according to a value in that block's field
data array, e.g., an initial condition, block ID, etc. Vector values
(arrays with more than one component) are fully supported.

Several other improvements were also made:

- Added test for existing capability of coloring by field data.

- Added test for coloring by tuple of field data.

- Modified vtkMapper::GetScalarModeAsString() to return
  string for field data scalar mode.

- Fixed vtkDataSetSurfaceFilter::UnstructuredGridExecute(...) and
  vtkUnstructuredGridGeometryFilter::RequestData(...)
  to pass field data through to output

Change-Id: I0d97e84ec77e7ea01ca1b1350acce73cb6c4c15b