Single track transport mode now possible

This MR introduces the possibility to run in single track mode to test benefits from stage locality. One can activate the line:

--config-single-track 1

in TestEm3_GV_ATLASbar.mac or FullCMS.mac. This transport mode takes bunches of tracks from the event server, but then propagates them one by one throughout the successive simulation stages. This mode can be compared with the scalar non-basketized mode to measure the impact of stage locality to the performance. The 2 modes should be profiled using our profiling tools to get a more clear picture.

Jenkins Build SUCCESS

Results available at: Jenkins [GeantV-gitlab #686]

merged