This change makes it possible to map vtkStringArrays to colors using
indexed lookup mode. The vtkStringArrays may be associated with
points, cells, or neither (field data). A slew of changes were
required to give this capability:

- Changed signature of vtkScalarsToColors::MapScalars(...) and
  vtkDiscretizableColorTransferFunction::MapScalars(...)  to take a
  vtkAbstractArray instead of a vtkDataArray. This enables mapping
  non-vtkDataArray arrays, such as vtkStringArrays. These changes are
  backwards compatible because vtkAbstractArray is the parent class of
  vtkDataArray.

- Changed vtkLookupTable::MapScalarsThroughTable2(...) to handle
  vtkStringArrays.

- Changed vtkColorTransferFunction::MapScalarsThroughTable2(...) to handle
  vtkStringArrays.

- Added vtkAbstractMapper::GetAbstractScalars(...) that returns a data
  array as a vtkAbstractArray - needed to retrieve vtkStringArray
  scalars.

- Added some error reporting when unhandled array ...