Remove references to old VTKData/Baseline directories

The VTKData repository no longer holds Baseline directories so drop
references to such directories from the VTK source tree:

* Replace the default -V in CMake/vtkTestingRenderingDriver.cmake with a
  value that will fail but indicate what to do.  All current uses of
  this module have an explicit -V so the default should never be used.

* Drop a few leftover Baseline directory references from comments in
  "*/*/Testing/*/CMakeLists.txt".

* Remove boilerplate duplicate test command-line examples (that
  reference Baseline directories) from *.py tests and leave an updated
  example in "Wrapping/Python/README.txt".  The command-line example
  convention was added long before the modern CTest infrastructure
  and is no longer needed anyway.

Change-Id: I3001e99b3cc8c2717107e61b3b7f988caeb49cab
BUG #14086: Fix vtkImageAppend when no active scalars are present.

vtkImageAppend failed when the image data didn't have any point-arrays marked as
scalars. Though vtkImageAppend's execution handled that case correctly, the
initialization did not. This patch fixes the initialization code.

Change-Id: I9207646b13038a96a26051fc3aed6d554a55aaf3
Merge topic 'fix-image-append' into master

710ce99d BUG #14086: Fix vtkImageAppend when no active scalars are present.
Merge topic 'cleanup-Baseline-references' into master

fc5d874c Remove references to old VTKData/Baseline directories