Sometimes the division between number of steps and time step was not
giving the right results.
Nevertheless, this just indicates that the species have to be in
separated arrays so that the assignment of particles in list, pushing
and scattering can be dealt independently.
Thus, this is the first step in creating separate arrays of particles
per species.
When the relative velocity between a charged particle and the background
for Coulomb collisions (W in the code) was low, there was a
segmentation fault. This is fixed now as if the norm of the relative
velocity (normW) in the code is too low, no collision is applied.
First attemp for Coulomb collisions based on the moments distribtuions.
Still the method is not done and far from being complete but input
options and basic math are implemented.
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.
