Ideal model of a chopper situated at Z=0. If a photon arrives at the chopper plane
at a time of t = [t0 +-n*T: t0 +-n*T +tau], where T is the period of the chopper, t0 the initial delay
and tau the opening time of the chopper, it is left untouched - otherwise it is ABSORBed.
If on a continous source the isfirst parameter may be used. In this case the photon time is _defined_
by the chopper. In other words no photons are absorbed, the photon time is merely sampled within the chopper window.
t_rise is the rise-time of the chopper opening giving a trapezoidal shape.
Limitations: this component does not take chopper geometry into account. If isfirst only samples in the first chopper opening window.
Parameters in boldface are required;
the others are optional.