Support writing of hex elements to VTKHDF format.

[pshriwise]

Description

This includes updates to the VTKHDF writer method so that its capabilities match those of the other VTK formats (.vtk/.vtu) supported by UnstructuredMesh.write_data_to_vtk).

Partially addresses #3620

Checklist

• I have performed a self-review of my own code
• I have run clang-format (version 15) on any C++ source files (if applicable)
• I have followed the style guidelines for Python source files (if applicable)
• I have made corresponding changes to the documentation (if applicable)
• I have added tests that prove my fix is effective or that my feature works (if applicable)

[pshriwise]

@jon-proximafusion I meant to mention to you a change in design from creating a dataset and appending to creation of the dataset with the data directly. Because we're working with statically sized datasets here (once we've determined the connectivity), creating them with the necessary size from the onset seemed to make the most sense to me.

Of course do let me know if that's not considered best practice for these files as the format is quite new to me.

[shimwell]

The code looks good to me @pshriwise
I think your redesign to creating arrays with the necessary size from the onset is a good one.
I've not been able to run this locally (I will need to install libmesh for that) so I was very happy to see you have extended the tests as well.
Tagging @mwestphal to show that we are continuing the hdfvtk which I think he will be happy with seeing the impact of his work.