ENH: Added more GetTransform methods to vtkMRMLTransformNode

A few get functions were missing (it was not difficult to compute the inverse but it was unnecessary extra code at many places):
GetTransformFromNode
GetMatrixTransformFromWorld
GetMatrixTransformFromNode

Added static method for getting concatenated transforms between any two transform nodes:
GetTransformBetweenNodes
GetMatrixTransformBetweenNodes

NULL nodes are accepted as input, for uniform handling of transformed and non-transformed nodes.
This method allows computation of transforms from one node's coordinate system to the other with a single line of code, regardless of what kind of transform (if any) are applied on each node.

git-svn-id: http://svn.slicer.org/Slicer4/trunk@24950 3bd1e089-480b-0410-8dfb-8563597acbee