Testing new VTU format.
For now, species information is ALWAYS output in .vtu (to test, this will
be an independent format in the future).
A .pvd file is produced to do time-series.
Still to implement other outputs (electromagnetic, average,
collisions...)
Still to implement reading a mesh from .vtu file
Now particles can be split to increase statistics when they enter a
smaller cell. However, this only has an effect for collisions.
Still, I have to rethink a lot about this feature.