Add a detailed description of wire digit time with distance of closest approach
Before this merge request, the sTGC digitization time depends only on the incident angle. It is a simple model, resulting in a poor sTGC timming. This merge request replaces it by a more detailed description that parametrizes time as a function of the distance between a hit and the closest wire. It is achieved 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.