vtkFixedPointVolumeRayCastMapper output is not consistent when the
number of threads exceeds 1. Although the differences are often
difficut to see, image comparisions show that the output varies as the
number of threads change.

This patch warns if the number of threads exceeds 1. It is up to the
application to decide whether differences images are worth the
trade-off for reproducibility.

Class documentation now describes the trade-offs regarding threads.

The tcl and python tests have been changed to keep the the number of
threads to 1.

NOTE: there is an outstanding bug ticket to look at the threading
defects. See:
http://www.vtk.org/Bug/view.php?id=13420

Change-Id: Ia33220fb9d6cf53250700e0fb8f1c5efe6c7e8c6