I should've commited before, but I wanted to make things compile.
The big change is that I've added a global time step so the parameter
does not need to be passed in each function. This is useful as we are
moving towards using time profiles for boundary conditions and injection
of particles (not in this branch, but in the future and the procedure
will be quite similar)
After some testing and making things a bit better and more general, I am
quite happy with the implementation of vtu and it seems that it is
working (at least as good as Gmsh2).
There are some procedures that might be useful for other XML-like
formats that might be moved in the future to the common module (I am
thinking right now in the implementation of a general format like
XDMF3).
