Extend VTKHDF file format to read/write higher order grids

[jorgensd]

• VTKHDF now supports reading and writing higher order grids (arbitrary order).
  • CoordinateElement for degree > 2 will be equispaced.
  • The support includes mixed topology.
    • Test using a mesh with second order quadrilateral and second order triangular cells have been added.
• Added special handling of 1D and 2D geometries (gdim argument in read constructor).
  • This is because VTKHDF doesn't have a notion of 2D geometries, in the same way XDMF supported XY points and XYZ points.

Partially addresses #3582

[garth-wells]

Needs some tests.

Would be good to make the title clearer - does it support write, read, or both?

[jorgensd]

Needs some tests.

Would be good to make the title clearer - does it support write, read, or both?

Do you have any good examples at hand for mixed meshes?
I can hand-code a 2D higher order mixed mesh (1 second order triangle + 1 second order quadrilateral).

The code supports read and write.

[chrisrichardson]

There is a mixed mesh fixture (see last test in io/test_vtkdhf.py) although this is only first-order.

[garth-wells]

Single cell type tests would be enough for now.

[jorgensd]

Tests for 1st, 2nd and 3rd order single cell meshes (tetrahedra) have been added, as well as a hand-coded 2-element mesh (2nd order triangle, 2nd quadrilateral).
They exposed a few flaws in the original design, including:

• VTKHDF doesn't have a notion of 2D grids (we therefore need to supply gdim (as we do when reading gmsh meshes) if we want a 1D or 2D grid
• For higher order grids (degree >2) a Lagrange type is needed. It has now been hardcoded to equispaced both in Python and C++, for consistency and is similar to what we do for GMSH grids.

Tests for higher order mixed dimensional mesh, and higher order single cell mesh added.