Use VTK Lagrange cells for ParaView output

[nilsvu]

Feature request:

VTK supports higher-order Lagrange cells, which we could use to have ParaView interpolate correctly between our data points, instead of the linear interpolation it currently does. Here's some resources:

• https://blog.kitware.com/modeling-arbitrary-order-lagrange-finite-elements-in-the-visualization-toolkit/
• https://github.com/dealii/dealii/issues/6931
• https://github.com/dealii/dealii/pull/6994
• https://github.com/GLVis/glvis/issues/64
• https://github.com/agrayver/VTK_Lagrange

[nilsdeppe]

This would be fantastic! Does XDMF have support for this yet? That's the only thing I can think of that would make things tricky. We already write the basis and quadrature, so for FD/FV we can use the cell-by-cell representation.