Remove pixel and voxel cell types
These cell types are inherited from VTK, but they are basically the same as quad and hexahedron, respectively. The only useful difference is that pixel and voxel are supposed to be axis aligned, but you cannot determine that by the cell shape alone (at least not just from the cell set). A big issue with these is that their indexing is different that of quad and hex. The development team had a long discussion about the benefits of the alternate indexing, but after consulting with Berk Geveci and Will Schroder from the VTK team, that indexing is not really taken advantage of at the cell level. Thus, it is really just a nuisance in VTK-m.
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