EVPFFT GUI Renders Voxelization with Misleading Scale

[kpwelsh]

Issue

When we render the voxel VTK we are going to give to the EVPFFT backend, the positions of the voxels are stored in coordinate space. This can be confusing for users, as the mesh they see can change scale upon voxelization. If they enter a voxel ratio of 1x2x1, for example, it will appear that their mesh was stretched by a factor of 2 along the y dimension. Additionally, the mesh is not centered, which makes it annoying to manipulate in the viewer.

Potential fixes

Lots of potential solutions

1. Let the EVPFFT backend handle real space coordinates and not index space coordinates. Then just ask the voxelizer to give the real coordinates.
2. Voxelize it twice, once for EVPFFT backend and once for the rendering. Little stupid, but trivial to do. Performance is probably still "fine".
3. Scale the resulting voxel grid in paraview.