Add counters for random PV position to lumi summary

Uses the index of a lumi event within its slice modulo the number of PVs in the event to select a pseudo-random PV, and saves the co-ordinates of that PV as lumi counters. All co-ordinates are saved to 10-bit counters (X and Y in um, Z in mm) with the counter offset by 512 to allow negative values in the unsigned counters.

FYI @sxian @egraveri @edallocc @balagura