fluence actor

[dsarrut]

First version

• still some copy/paste from DoseActor. To be improved
• no option yet, still simple hits counters
• future options : scatter, uncertainty, energy_model

[andiresch]

as an option: the fluence could also be retrieved from the LETActor.cc enabling the track_average option (denominator). There we use the sum of all track lengths, which still would need to be divided by the volume to result in the fluence (this division is not needed in LET_fluence calculation as it cancels out from the fraction as it is in the denominator and numerator) https://iopscience.iop.org/article/10.1088/0031-9155/39/6/011/pdf