70 x premixing si strip noise fix

So, in discussions with Tommaso B., it appears that the previous strategy for handling noise generation in the PreMixing was wrong. (He would know, having written the original code.) This fixes that problem by incorporating noise and zero suppression into the DataMixing step.  Better to correct this now than later!

Code modifications: 
1) Added a new module to the DataMixer to specifically treat Digi combination and noise generation in MC Strip digis.
2) Reorganization of SimTracker/SiStripDigitizer so that lower-level routines can be used outside of this package (by the DataMixer). No code changes, just locations of code/header files.  
3) Factorization of SiStrip digitization parameters in SimGeneral/MixingModule so that the parameters themselves can be imported by the DataMixer independent of the Producer definitions.