add dndx calculation
add dndx calculation in Reconstruction/ParticleID/, TPCDndxAlg. fix bugs in Digitization/DigiSimple, TPCPixelDigiAlg & TPCPixelDigiTool. Merge hits on the same pxl from different clusters into one hit.
add dndx calculation in Reconstruction/ParticleID/, TPCDndxAlg. fix bugs in Digitization/DigiSimple, TPCPixelDigiAlg & TPCPixelDigiTool. Merge hits on the same pxl from different clusters into one hit.