Add a detailed description of wire digit time with distance of closest approach
Before this merge request, the sTGC digitization time is independent of the distance between the hit and the closest wire. It is a simple model, resulting in to a poor sTGC timming. This merge request replaces it by a more detailed description that parametrizes time as a function of the distance between hit and closest wire. The characterization of time is done by dividing the distance into 9 intervals of 0.1 mm and performing a fit of the time distribution in each interval. It is found that digit time follows a gamma distribution function. So the implementation uses the CLHEP RandGamma random number generator.