Fix bug with voxels in legacy vtk files
The legacy VTK file reader for unstructured grids had a bug when reading cells of type voxel. VTK-m does not support the voxel cell type in unstructured grids (i.e. explicit cell sets), so it has to convert them to hexahedron cells. A bug in the reader was mangling the cell array index during this conversion.
Fixes #711 (closed)