gen_amp_V2 vertex centering

[jrstevenjlab]

Add vertex centering as default for gen_amp_V2, which places all generated events at the center of the target (x,y,z) = (0,0,65) cm. This is the default for most generators, which leaves the simulation of the vertex distribution to hdgeant4. When these events are processed by hdgeant4 it first spreads the event vertices randomly across the target volume by default to get the appropriate distribution along the beamline.

Thanks to @duberii for identifying this issue

[gluex]

Test status for this pull request: SUCCESS

Summary: summary.txt
Logs: results/log

Build log: make_gen_amp_V2_centerVertex.log
Build report: report_gen_amp_V2_centerVertex.txt

[mashephe]

Thanks for making this change to make compliant with standard behavior.