Add virtual refinement for VTK output

Description

Right now, we only write data at the vertices of the triangulation, this naturally correspond to first order polynomial finite elements. Since vtk does not ha a good way to write arbitrary polynomial order and/or is not convenient for a software like dune, we opt for the old option to fake local refinement in order to attach more data to a single real entity.

Proposal

Add and option in the configuration file to set the virtual refinement level and use the SubsamplingVTKWriter provided by dune-grid which was specifically made for this task.

How to test the implementation?

Write a python wrapper that checks that higher virtual refinement level produces bigger files for the first step.

Related issues

See #13 (closed)