I have made some changes to make itk::PathToImageFilter to make it a usable base class. There were
some problems with the CVS version, including: (1) this ase classhad actual filter functionality, (2)
the mechanism for specifying the size, spacing, origin, etc., of the output image was incomplete, contrary
to ITK convention, and hard to extend, and (3) the documentation specified that the filter would calculate
the size of the output image from the path, but it did not.

I solved these problems by (1) moving the filter functionality to itk::TracePathOnImageFilter, (2)
requiring that filter->GetOutput()->Set...(...) should be used to set the information for the
output image, and (3) providing methods to calculate the correct image region a path inhabits, and a
boolean flag to specify whether that calculation should be done (default true).