PluginElements: particles not recorded when element crosses (or close to) origin

Summary

Noticed by @nesteruk_k: Particles are not recorded in some Probes. Likely the other PluginElements are affected too.

Steps to reproduce

A Probe crossing the origin, e.g. one defined as:

P: Probe, XSTART=-1e10, YSTART=0, XEND=1e10, YEND=0;

will not record any particles.

What is the expected correct behavior?

Recorded particle and output files: P.hist, P.h5, P.peaks

Possible fixes

A check is performed if the bunch is close to the probe (and only then individual particles are checked) as follows:

https://gitlab.psi.ch/OPAL/src/blob/master/src/Classic/AbsBeamline/Probe.cpp#L62

if( rbunch_max > rstart_m - 10.0 && rbunch_min < rend_m + 10.0 ) {

With rstart_m and rend_m defined as:

https://gitlab.psi.ch/OPAL/src/blob/master/src/Classic/AbsBeamline/PluginElement.cpp#L86

    rstart_m = std::hypot(xstart, ystart);
    rend_m   = std::hypot(xend,   yend);
    // start position is the one with lowest radius
    if (rstart_m > rend_m) {
        std::swap(xstart_m, xend_m);
        std::swap(ystart_m, yend_m);
        std::swap(rstart_m, rend_m);
    }

Instead of rstart_m the closest point to the origin should be used in the check.