Mergerequest 7 27 16

adding: vtkOSPRayVolumeMapper - a volume mapper than can be used inside of an OpenGL rendering context. Blends and composites with existing framebuffer. Modified to inherit from volummapper base class vtkOSPRayVolumeMapperNode - volume rendering implementation in scenegraph structure vtkOSPRayVolumeNode - an ospray volume node in the scene graph vtkVolumeNode - a volume node in the scene graph

modified: vtkOSPRayRendererNode - composites with existing framebuffer when the background is not being erased. various opengl code for supporting ospray in the smartvolumemappers c++11 support required for ospray module. If this is too burdensome, this dependency can be take out. paraview modifications: not included in this merge, but the only change to paraview is to select ospray rendermode for the volume renderer. Optionally, ospray volume specific options such as sampling rate can be added. Currently, this is compute automatically.