Bugfix/track elem by elem rolling

[martinschwinzerl]

• Fixes a logical error preventing the is_rolling flag of NS(ElemByElemConfig) from properly working
• Fixes an initialization issue that may have prevented the initialization of TrackJobs with until_turn_elem_by_elem=1 from properly working

Note: On NVIDIA OpenCL implementations, please make sure to delete the compile-cache (i.e.

rm -rf ~/.nv/ComputeCache/*

; on my system at least, the runtime compiler was not able to pick up the changes to the included header-only parts of SixTrackLib otherwise

[martinschwinzerl]

@rdemaria: could you please verify whether this changes fix the track_elem_by_elem issues encountered earlier today?

It should now be possible to initialize and track until turn 1 and to use the rolling feature (which is on by default). Thank you!