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.
I have set the velocity of the particle to be a dummy Maxwellian
distribution to start testing things.
Also I added a constant yield table for testing.
Next step is to do the user input and run some tests.
