VTKHDF mesh I/O

[chrisrichardson]

Adds a simple reader and writer for Mesh using the VTKHDF format.
This is currently limited to first order meshes (but can be extended to higher order).
Includes support for mixed-topology meshes.

• Add float32 support in Python layer
• Add Python wrapper
• Add mixed-topology test fixture

[jorgensd]

Why have you chosen to restrict it to first order meshes (the restriction is not present in my Python implementation)

[chrisrichardson]

@jorgensd - partly because higher order pyramids are not working in Paraview, partly just to keep it simpler for now.
I noted above that it can be an extension...

[jorgensd]

@jorgensd - partly because higher order pyramids are not working in Paraview, partly just to keep it simpler for now. I noted above that it can be an extension...

I really think we should include higher order right away.
XDMF only supports second order grids. Thus we have no API for reading in higher order meshes that are not generated through GMSH (max degree 3).

very few (if any uses pyramids atm, especially higher order ones, and if the issue is with Paraview it shouldn’t limit us).

[jorgensd]

Additionally, the code complexity for higher order is not really hard (as shown in the mixed topology io branch with the Python implementation).