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. 
Actually, compose2 also is used in some other functions, like GetFirstHitTimeInGate, which is needed. compose2 allows you to look through a collection and remove all elements which are less than the lower range and greater than the upper range. There are probably other ways to do this, but this is consistant with how we treat the hits collections in the code.