shadow thermal load: add final spectrum plot

[srio]

It would be very useful to add a plot with the final spectrum (number of photons vs energy), and may be other plots:

PS: I can do it!

[lucarebuffi]

It is already done in the source (Tab "Cumulated Plots"). Despite the fact that engineers never asked me that information, it is actually useful, even simply as a diagnostic of the simulation. I'll do it, no problem.