BUG 13603: make vtkContourFilter properly push progress notifications

vtkContourFilter uses several specialized subfilters depending
on the input data type to perform the contouring. However, the
progress from these subfilters wasn't pushed through the
vtkContourFilter object. This patch installs an observer on
the subfilter and properly updates vtkContourFilter's progress.

The work for this patch has been sponsored by BP.

Change-Id: I849b442318b0a59a64099add2fa6ccd74967725a
Merge topic 'vtkContourFilter-progress-fix' into master

ecaddd46 BUG 13603: make vtkContourFilter properly push progress notifications