Improved hit converter and event display

[dguff]

• hit_converter bugfix: When a TPC is not centered in the origin of the axes, the position of the anode is not correct as it is defined with respect to the TPC center. Now the global TPC offset is accounted
• hit_viewer: the hit_viewer now displays also MC truth tracks. MC truth particles can be turned on and off via GUI and it is possible to define energy threshold for displaying the particle trajectories.