Corrected ApplyResponseToTrackerDigit():

Corrected ApplyResponseToTrackerDigit():
- the de-calibration is applied on the physics signal (then at
  digit::Signal() )
- noise is independent of the gain. (It is a constant in the dynamics range
  of the electronics. Indeed, pedestal data is the results from the FEE and
  gain is been already taken account. Then the noise need simply to be added
  to digit::signal/gainMean.
- pedestal is added after.
Then acd=(signal/gainMean) + adcNoise + pedestal if we have a physic signal
and adc=adcNoise+Pedestal if it is a noisy pads.
(Alexandre)