Bad null pointer check in vtkBSplineTransform.

The ForwardTransformPoint method was missing a "!" in a null pointer
check, causing the transform to perform an identity transform.  The
tests for this class did not cover this code path, even though it is
the most common path in the general usage of this class.  A new test
has been added, and an old test has been re-enabled.

Also, add the missing GetDisplacementScale method.

Change-Id: I3cc21fda11e4c4f0a83dc020e0e9968086246d45
Merge topic 'bspline-transform-fix' into master

ddf898ae Bad null pointer check in vtkBSplineTransform.