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.
The following test cases:
- 04-JT60SA
- 05-JT60SA-startup
- 06-ITER-startup.Mixed
were affected by the bug fixed in f82f91bc, so their reference outputs
had to be updated.
