Enable vtkmSlice for 3D image data
There is a condition in vtkmSlice.cxx
that prevents it from using VTK-m when processing 3D image data (https://gitlab.kitware.com/vtk/vtk/-/blob/master/Accelerators/Vtkm/Filters/vtkmSlice.cxx#L61). This is because there used to be a bug in VTK-m's flying edges for some corner cases. This problem has been fixed, and vtkmSlice
should use VTK-m for this condition.