Support GMSH/VTK prism/pyramid cells

[tiagovla]

I noticed the cpp io interface does not yet support GMSH/VTK prism/pyramid cells which might be needed to support hybrid 3d meshes in the future. Could I make PR to contribute on that?

• [ ] Add logic to cells.cpp
• [ ] Add documentation to cells.h
• [ ] Add tests (maybe test fixtures)

[jorgensd]

@tiagovla Feel free to add such a contribution to the code. As far as I can tell by quickly looking at: https://raw.githubusercontent.com/Kitware/vtk-examples/gh-pages/src/Testing/Baseline/Cxx/GeometricObjects/TestLinearCellDemo.png and https://github.com/FEniCS/basix#prism The map for a first order prism/wedge is the identity: [0,1,2,3,4,5] while the map for a pyramid is : [0,1,2,3,2,4]. Feel free to try to add these for vtk and check the correctness by writing it to pvd (VTKFile).

[garth-wells]

I'd suggest waiting on this until we have the low-level plumbing in place for mixed topologies.