XDMF FiniteElementFunction format
This merge request is a part of work of my GSoC project https://summerofcode.withgoogle.com/dashboard/project/5552949523644416/overview/
The idea of the project is to introduce format for finite element function's storage in XDMF and their visualization in Paraview. There are two main differences to how VTK and XDMF format works now:
- we use two data array format where first array define cell-wise indices to the second array of values
- we allow not only isoparametric, but also subparametric and superparametric elements to be visualized
More about definition in XDMF to be found in http://www.xdmf.org/index.php/XDMF_Model_and_Format#Attribute
In summary, this MR influence only xdmf->vtk translation layer. It rebuilds mesh when superparametric element. This type of format allows also more exotic finite elements to be visualized - this is demonstrated in Raviart-Thomas element.