LArCondTPCnv: Allow for FEBs with nChannels not being a mutiple of 32 (like SuperCells "FEBs")
The current implementation of the T/P converters for LAr Electronic Calibration values implicitly assumed that the number of channels of each FEB is a multiple in 32 (128, in practice). This assumption doesn't hold for SuperCells which have 312 channels per board. This MR should fix this shortcoming.