ENH: Moved ghost level array generation in structured output from duplicated UpdateData methods in the structured data objects to a single GenerateGhostLevelArray method in vtkDataSet.  This is now possible because the extent is stored in the information object on each data object.  This code is now called by vtkSource at the end of UpdateData.  Similarly the call to Crop() was moved from vtkDataObject::UpdateData to the end of vtkSource::UpdateData.