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. 
So, it looks to still be true that this isn't part of the C++ standard and won't go through the compiler. However, I can get away with not defining all of the other using __gnu things (cxx::identity_element, etc), so this section from line 21-42 could be deleted with the exception of using __gnu_cxx::compose2;.