Add overflow check to eTower summation (tau seed position fix)

In events with saturated supercells mismatches were seen between hardware output and simulation. These were caused by the simulation not checking for cell overflows when forming full tower sums for the seeding algorithm. This change adds an overflow test to the function in eTower that sums all cells in the tower (it does resolve the problem in the lb I was testing it in).

(Fixes issue seen in run 474271 lb 593 and 594 eTAU tobs that are saturated but not in the location that matches the fw (neighbouring towers had saturated cells, so should be treated equally).)