mixed meshes

I wonder what are good ways to treat mixed meshes; e.g. a combination of triangles and quadrilaterals. I recently received one with wedges #411 and tetrahedra, but I would envisage treating the tri–quad case first. One of these arose by accident in one of the examples when Gmsh failed to combine all the triangles into quadrilaterals. #288

I wonder whether this could be done with the library as is, using multiple meshes with the same points, a bit like subdomains (but not literal Mesh.subdomains); a meshio.Mesh can support blocks of cells of different types on the one array of points.

[kinnala]

If you have several of these tuples (Mesh, Element) and some kind of pair (BasisType, Form) you should be assemble the meshes and the elements at one go. E.g., me = [(MeshTri(...), ElementTriP1()), (MeshQuad(...), ElementQuad1())] (meshes may share indices) and then A = multiasm(me, CellBasis, laplace) would work out the parallelization and fixing Dofs objects and combining the result.

[kinnala]

I was thinking this while working on #400 but didn’t yet do anything about it. I think the correct way to do it is to create multiple Mesh-like entities that share the indices of the corresponding nodes. Then the rest of the machinery from InteriorBasis to BilinearForm should detect that there are multiple meshes and then ultimately try to combine them into a single matrix. But it requires some major changes, I could keep this in mind when continuing from #400 .