Fix to select correct first sample of digits for raw channel building

This fix ensures that the correct first sample is chosen for raw channel reconstruction when preceding samples are saved in the digits vector. Ref: https://indico.cern.ch/event/1204518/contributions/5079149/attachments/2521134/4335073/NS_LArWeek_03_10_2022.pdf The difference between reconstructed energies before and after the fix is due to the different random numbers used for generating electronic noise during creation of the digits vector of different sizes (backup slide), as is evident from the Gaussian shape of ΔE