This makes the following changes to summary.7.txt:
1. Preserve the sign of J_φ_peak J_φ_max, i.e. if the J_φ is negative,
those will be negative. Previously they would always be unsigned.
2. If the sign of J_φ is alternating significantly, specifically if
(∫|J_φ|dA - |∫J_φ⋅dA|) / ∫|J_φ|dA > 10%
then sign of ρ_avg_J, Δρ_avg_J, ρ_max_J, Δρ_max_J will be negative.
Previously they were always positive.
3. If there is no current J_φ=0 (power dPdV=0), then
ρ_avg_J=1, rho_max_J=1 (ρ_avg_P=1, rho_max_P=1).
Previosuly they would be set to zero.
This helps producing continuous results as a beam is gradually
focused into the plasma.
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.
