VTKHDF file format: cell type in PolyData should be optional
Context
Currently, if someone define a PolyData in a VTKHDF file, even if the polydata will contain only polygon, this file needs to add the structure for each other cell types.
In this example the dataset should have only polygon but needs to define Lines
, Vertices
etc...
Solution
We may want to update the VTKHDF file format to make these groups optional.
Note that this change can have some impact for the Transient data file format as we do some assumption when we compute the cell offset.