feat: allow asymmetric tracking region, default to symmetric

[wdconinc]

At the EIC (and other asymmetric colliders) the detector layout is asymmetric. Thus, we wish to define asymmetric tracking regions. This PR introduces tracker_region_zmin which is the (signed) negative z position of the tracking region. If not specified, it is set to the negative of zmax and an info message is printed (ensuring backwards compatibility).

BEGINRELEASENOTES

• DDG4: Geant4TCUserParticleHandler: Allow for asymmetric tracking region with tracker_region_zmin

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[andresailer]

Looks good to me. The only thing I am wondering about is the new info print out, if that should be debug. But if no one objects I'll merge next week...

[wdconinc]

I'll change it to debug. I was also looking for other docs on this tracking region functionality, but couldn't find it. Are there any other locations it can be included under docs/?

[andresailer]

There is some description in https://dd4hep.web.cern.ch/dd4hep/usermanuals/DDG4Manual/DDG4Manual.pdf section 5.3 Monte-Carlo Truth Handling

[wdconinc]

I finally had some time to look at the section of the docs. It is more general in scope, not specifically about Geant4TCUserParticleHandler, so adding this asymmetric tracking region would require describing also this particular implementation of a Geant4UserParticleHandler. Therefore, I think it is probably fine to leave this as is.