More efficient PrincipalAxisNode rendering

PrincpialAxisNode (ACG/Scenegraph/PrincipalAxisNode.cc) is rendered in a rather inefficient manner: Axis nodes consist of only 3 colors but every vertex has its individual color. That's some major waste of GPU memory. At the point of this writing, the file in question is not yet merged into master but in branch move_principal_axis_node_from_physim.