This commit implements some structural changes to the code. The information for a PMT (name, pe response, geometry, and QE) now is all contained together in the src/WCSimPMTObject.cc file. PMTs are a member of the abstract base class WCSimPMTObject and the virtual functions for this class can be found in include/WCSimPMTObject.hh. PMTs are now instantiated in the src/WCSimDetectorConfigs.cc file and a pointer to that PMT type is stored, allowing for the pe response and QE to be accessed by the appropriate files.
Adding timingConstant in WCSimWCDigitizer to the information stored with the PMT.
Instead of just defining the timing constant which is used to calculate the timing resolution, the user can now define the entire timing resolution function. This is to allow for the parameters of the timing resolution function to be modified for the new HPD tubes, but can also be used in the future if the timing resolution function has a different structure.
Adding WCPMTGlassThickness to the list of properties defined in src/WCSimPMTProperties.cc.
Merge pull request #30 from WCSim/feature/issue12

Merging the PMT implementation change from Erin.  This defines all PMT properties and responses in their own object.