Add VR CameraPose to vtkVRCamera

Add a VR camera pose that captures VR specific properties and provides methods to save/apply such poses.

To reproduce a pose using the current camera and render window means to try to make the viewers current view look like the original saved pose. This is complicated by the fact that the viewer may now occupy a very different position and orientation in the physical space than when the pose was saved. This method accounts for this and adjusts the physical space to best fit the requested pose.