ENH: Update vtkDelaunay2D to support DesiredOutputPrecision

This updates vtkDelaunay2D to support a choice of desired precision
(default, single, or double) for output points. It uses the analogous
code in vtkThreshold as a model.

Change-Id: Ib5d9149f5bb34fc4be0b5005b0e94e38fcf5b52b