Properly get bin edge info from TH2F in PFClient

ROOT 5.34.17 gives an error (which is converted to an exception) if
GetBinLowEdge called directly on a TH2F. The correct way is to call
that method on the result of TH2F::GetXaxis().
Merge pull request #2902 from Dr15Jones/fixBadTH2FcallInPFClient

Multithreading -- Properly get bin edge info from TH2F in PFClient