Change ParticleWithBremMaker's counter and make message more clear
This changes the counter type of the number of particles processed to a simple Counter, as the StatCounter didn't make much sense and led to a confusing interpretation, eg:
ParticleWithBremMaker_da6ec954 INFO Number of counters : 1
| Counter | # | sum | mean/eff^* | rms/err^* | min | max |
| "Nb of particles corrected with brem" | 100654 | 0 | 0.0000 | 0.0000 | 4.2950e+09 | 0.0000 |
By the way, it could be useful to pass buffers of counters to the SelectiveBremAdder as these counters are updated per particle in each event.