Fixed an issue in which some particles in the corner were interacting
with the axis boundary. Now the axis acts as a reflective boundary in
case a particle is wrongly assigned to it.
A boundary condition for each species must be indicated in the case
file.
This opens the door to use boundary conditions with different parameters
(for example, a wall temperature, coefficients for reflection or
absorption...)
The examples included with the code have been updated accordently.
