Enh: Clarify that GroupSO replaces SceneSO

Detail that GroupSpatialObject in ITKv5 replaces SceneSpatialObject,
and retains nearly identical functionality.

Author: aylward

SoftwareGuide/Latex/Architecture/SpatialObjects.tex

@@ -112,6 +112,9 @@ \subsection{GaussianSpatialObject}
 \subsection{GroupSpatialObject}
 \label{sec:GroupSpatialObject}
 \input{GroupSpatialObject.tex}
+
+With ITKv5, the \code{GroupSpatialObject} also replaces the
+ \code{SceneSpatialObject}. Much of the functionality is unchanged.
 \input{SceneSpatialObject.tex}