[DF] Add support for TBranchObjects (e.g. branches containing TH2F)

First reported here: RDataFrame throws an exception at RLoopManager.cxx:130, inside GetBranchNamesImpl, while retrieving the names of all available TTree branches, in presence of a TBranchObject: this case is simply not handled at the moment.

A ROOT file with a TBranchObject "value" containing a TH2F is available here.