proof: add integration for proof/proofbench in a dedicated PROOF subgroup

proof/proofbench/src/TProofBench.cxx

proof/proofbench/src/TProofBenchDataSet.cxx

proof/proofbench/src/TProofBenchRun.cxx

proof/proofbench/src/TProofBenchRunCPU.cxx

proof/proofbench/src/TProofBenchRunDataRead.cxx

proof/proofbench/src/TProofNodes.cxx

proof/proofbench/src/TProofPerfAnalysis.cxx

proof/proofbench/src/TSelEvent.cxx

proof/proofbench/src/TSelEventGen.cxx

proof/proofbench/src/TSelHandleDataSet.cxx

proof/proofbench/src/TSelHist.cxx