A simple unit test instrument for the Detector_pn component. It consists merely of
two sets of monitor each with:
1 regular PSD_monitor and 2 instances of Detector_pn with a scinitillator screen of
Si and Pb respectively. Obviously, no sane person would use Pb, but it's there for
illustration purposes.
A restore_flag parameter may be passed to Detector_pn which negates the absorption
effect for subsequent components.