bump VTK to OSPRay 3 API
Change CMake Find to look for 3. Stop using previously deprecated OSPRay APIs which are now removed in 3.0. Use new parameter signatures including: uint32 for enums box1f instead of vec2f for ranges "valueRanges" TransferFunction parameter is now "value". Implicit isocontours now take their appearance settings from a GeometricModel instead of a VolumetricModel.