Properly get bin edge info from TH2F in PFClient

[Dr15Jones]

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().

[Dr15Jones]

@ktf This is needed to fix many failures in _THREADED. I've test compiled it and ran one failing runTheMatrix (15.0) and everything works fine.

[cmsbuild]

A new Pull Request was created by @Dr15Jones (Chris Jones) for CMSSW_7_1_X.

Properly get bin edge info from TH2F in PFClient

It involves the following packages:

DQMOffline/PFTau

@ojeda, @danduggan, @rovere, @cmsbuild, @nclopezo, @deguio, @Degano can you please review it and eventually sign? Thanks.
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@nclopezo, @ktf you are the release manager for this.
You can merge this pull request by typing 'merge' in the first line of your comment.

[ktf]

Merging this seems seems to be safe enough. @deguio complain if not ok.

@Dr15Jones I assume th->GetXaxis() != 0 always, right?

[Dr15Jones]

The line right after it seems to assume that.