Add global flag to set NLL evaluation backend

  * In case the value is `"combine"`, it will use combines caching NLL
    classes

  * In any other case, it will RooFits default `createNLL()`, setting
    the RooFit::EvalBackend to the value on the global flag.
    See: https://root.cern.ch/doc/master/classRooAbsPdf.html#a24b1afec4fd149e08967eac4285800de

interface/Combine.h

@@ -32,6 +32,7 @@ extern bool bypassFrequentistFit_;
 extern  std::string setPhysicsModelParameterExpression_;
 extern  std::string setPhysicsModelParameterRangeExpression_;
 extern  std::string defineBackgroundOnlyModelParameterExpression_;
+extern  std::string nllBackend_;
 
 namespace { 
     struct ToCleanUp {

src/Combine.cc

@@ -91,6 +91,7 @@ TTree *Combine::tree_ = 0;
 std::string setPhysicsModelParameterExpression_ = "";
 std::string setPhysicsModelParameterRangeExpression_ = "";
 std::string defineBackgroundOnlyModelParameterExpression_ = "";
+std::string nllBackend_ = "combine";
 
 std::string Combine::trackParametersNameString_="";
 std::string Combine::trackErrorsNameString_="";

src/RooSimultaneousOpt.cc

@@ -2,6 +2,8 @@
 #include "../interface/CachingNLL.h"
 #include <RooCmdConfig.h>
 
+extern  std::string nllBackend_;
+
 #if ROOT_VERSION_CODE < ROOT_VERSION(6,30,0)
 RooAbsReal* 
 RooSimultaneousOpt::createNLL(RooAbsData& data, const RooLinkedList& cmdList) 
@@ -10,6 +12,17 @@ std::unique_ptr<RooAbsReal>
 RooSimultaneousOpt::createNLLImpl(RooAbsData& data, const RooLinkedList& cmdList) 
 #endif
 {
+    // Alternative evaluation backends are only supported from ROOT 6.30.
+#if ROOT_VERSION_CODE >= ROOT_VERSION(6,30,0)
+    if (nllBackend_ != "combine") {
+        RooLinkedList cmdListCopy{cmdList};
+        // Set the RooFit::EvalBackend() of RooFit to what the nllBackend_ in
+        // combine is set.
+        RooCmdArg evalBackendArg{RooFit::EvalBackend(nllBackend_)};
+        cmdListCopy.Add(&evalBackendArg);
+        return RooSimultaneous::createNLLImpl(data, cmdListCopy);
+    }
+#endif
     RooCmdConfig pc(Form("RooSimultaneousOpt::createNLL(%s)",GetName())) ;
     pc.defineSet("cPars","Constrain",0,0);
     RooArgSet *cPars = pc.getSet("cPars");