Fix a crash when deleting vtk-python objects.

This fixes a bug that has been present since commit 269cbea1 on Oct 16,
2013 where deleting an object that had a DeleteEvent observer would cause
a crash (the crash was only verified to occur on OS X).

Change-Id: I89d7a9a6eff9ab11fdcd7291a42f47b9a8017091
Merge topic 'python-ptr-erase' into master

cbd4884d Fix a crash when deleting vtk-python objects.
Redesign "vtkGhostLevels" arrays and related ghost functionalties.

 -The semantics of each unsigned char in the ghost arrays changes:
  Instead of storing a numeric value representing how far a cell is
  from the boundary, it is now a bit field specified by
  vtkDataSetAttributes::CellGhostTypes and
  vtkDataSetAttributes::PointGhostTypes.  The bit field is consistent
  with VisIt specs.

- Previously, filters strip all ghost cells they request from upstream
  before finalizing the output. This is no longer done.

- vtkUniform grids previously supports blanking through member arrays
  vtkUniformGrid::CellVisibility and
  vtkUniformGrid::PointVisibility. These arrays are removed and the
  blanking functionality are supported through the new ghost arrays
  instead.

- the "vtkGhostLevel" arrays for cell and point data are renamed by
  vtkDataSetAttributes::GhostArrayName().

Change-Id: If5ff516c0fb0b321aa9e0fcc8cf7e74f05e2b250