adding locally tested version of PlotFebAverageTime() to LAr TimingClass
The function PlotFebAverageTime() in TimingClass.h and TimingClass.cxx is not updated for the use of the FEB SImpleTiming.C local script that runs in larmon, therefore for now I am adding the locally used version under the name PlotFebAverageTime24(), in order not to run over the original version in case used elsewhere.
In the future after some verifications, we might remove the older one.