Probes are now written at the 0 iteration.
Additionally, and this shouldn't be done, some small changes to the quad
elements. This should be done in a separate commit, but I'm lazy.
Now, probes check all particles in the domain.
This is done only when probes are outputed to save CPU time.
However, still some issues and distribution functions are not properly
being calculated.
The code nows offer the possibility to obtain the distribution function
for a specific species in a 3D velocity grid at a determined position.
This is a simple method that just scatter the particles in one cell into
the velocity grid.
