Ray tracing - use voxelization #75
Description
It should be possible to make the ray tracing much faster if we use a voxelization for the local cell search.
The basic idea will be to map the OpenFOAM grid to a structured grid of cubes/voxels and store the maps. The ray tracing can then be performed on the voxels and then mapped back as needed.
The slow part of our current ray tracing is the local cell search, where we geometrically check if the ray is inside a cell: this requires multiple floating-point operations, as we check if the ray is inside each face. If we switch to voxels, we can do simple float comparisons, e.g. x_ray < voxel_max_x.