A volume that is compile-time sized and has its lower bound at (0, 0, 0).
The name RasterableVol was chosen because such a volume can be used with
VolGrid3d.
A volume that is compile-time sized in x and y direction and has its lower
bound at (0, 0, z). In z direction there’s no restriction on the lower
or upper bound. The name RectRasterableVol was chosen because such a
volume can be used with VolGrid2d.