vtkHDFReader: bad support of transient UnstructuredGrid with varying cells
When the number of cells changes between different timestep, cells in the dataset aren't correctly read by the vtkHDFReader but this use case is supported by the file format.
For example, this data is correct based on the VTKHDF file format : lines.vtkhdf. We expect to have 3 cells (t0) then 4 cells (for t1 and t2), but we always have 3 cells at each timestep.