1- Run Header
   ----------
   - NbOfEvents   N1Pixels   N2Pixels   N1Shift
   
2- loop on Events
   --------------
   - EventNb
   - Energy (MeV)   Direction   Position (mm)
   - NbOfPixels
   
   - loop on Pixels
     --------------
     - PixelNb (coded)   Evis (MeV)   Etot (MeV)   

Coding convention
-----------------
- Pixel has a double index :
    Pixel[i1,i2]  i1 = 0,...,N1Pixels-1,  i2 = 0,...,N2Pixels-1
- by convention PixelNb = i1*N1Shift+i2
- therefore : i1 = int(PixelNb/N1Shift),  i2 = PixelNb%N1Shift