Feature/issue4

This feature branch separates the PMT and digitizer function in WCSim. This will allow for other PMT types to be added and swapped out easily. The pe to charge correction was removed from WCSimWCDigitizer and a new file, WCSimWCPMT was created to do this routine. All of the routines in WCSimWCDigitizer were modifed to accept a digitized collection rather than a hits collection. The AddPMTDarkRate routine was modified to properly write out random noise to the digits collection. 
What does compose2 do?