Initialize sTgcDigitMaker once per sTgcDigitizationTool class declaration

Moving the sTgcDigitMaker initialization to right after the class declaration in the sTgcDigitizationTool::initialize method, so that sTgcDigitMaker is initialized only once. Since the parameters read from files during sTgcDigitMaker initialization are the same, there is no reason to re-read the files at each event. Also, a random engine is not required by the sTgcDigitMaker::initialize method. So the member object HepRandomEngine is deleted and, instead, a pointer to an element of IAthRNGSvc is used when shooting random numbers. This modification was suggested, see comments at MR !46472 (merged).