Special mapper for passing vtkmDataSet through for rendering

This change introduces a vtkmDataSetMapper whose role is to map vtkmDataSets to graphics primitives. As of now, vtkmDataSetMapper is just a no-op implementation that gets overriden when OSPRay is enabled. In that case, the OSPRay view node factory replaces the mapper with the vtkmOSPRayDataSetMapperNode that will hand over the vtkmDataSet directly to OSPRay for rendering without offloading from the GPU device.

Another critical aspect of this change is that the vtkVolumeMapper now supports API to accept vtkmDataSet as input. This allows the OSPRay volume mapper node to render vtkm datasets directly.