Read mixed topology mesh from file

[chrisrichardson]

Add functionality to read a mixed topology mesh (e.g. triangle + quad, hex+prism+pyramid+tet) from file.

• file format? VTK, XDMF, ADIOS2

[garth-wells]

• file format? VTK, XDMF, ADIOS2

I'd suggest VTKHDF, with maybe VTK first because it's easy to read.

[jorgensd]

If we go for VTKHDF, please consider: https://discourse.vtk.org/t/vtkhdf-point-ownership/13913/4
as it is to me unclear how to actually make use of the partitioning offsets in VTKHDF.
https://gist.github.com/jorgensd/6b3db748009ac1017929d7e7f3e83dc2
shows how the data-structure is written for a single cell type, which is fairly easy reverse engineer to reading and/or mixed topology.

[chrisrichardson]

Thanks @jorgensd - I will take a look.