Store the polarisation ellipse angles χ, ψ only for the central ray.
Otherwise they'll be zeroed out since `plasma_in` does not compute the
polarisation of the other rays.
This changes the ψ, χ polarisation ellipse angles shown in the summary
table to be those of the current plasma wave mode, not the polarisation
of the beam upon re-entering the plasma boundary after reflecting.
Note that the former has always been the intended value, but was likely
changed inadvertently when reflections have been implemented.
