[df] Introduce function to retrieve type of data source

Introduce a new method to get a label for the data source that the current RDataFrame is processing. There are three main types:
* The dataframe will process a TTree dataset
* The dataframe will process an empty dataset
* The dataframe will process data from an RDataSource

The function returns a label with the suffix "DS" also for the first two cases, to be aligned as much as possible with the RDataSource infrastructure.

tree/dataframe/inc/ROOT/RDF/RInterface.hxx

@@ -89,6 +89,7 @@ class GraphCreatorHelper;
 void ChangeEmptyEntryRange(const ROOT::RDF::RNode &node, std::pair<ULong64_t, ULong64_t> &&newRange);
 void ChangeSpec(const ROOT::RDF::RNode &node, ROOT::RDF::Experimental::RDatasetSpec &&spec);
 void TriggerRun(ROOT::RDF::RNode node);
+std::string GetDataSourceLabel(const ROOT::RDF::RNode &node);
 } // namespace RDF
 } // namespace Internal
 
@@ -121,7 +122,7 @@ class RInterface : public RInterfaceBase {
    friend void RDFInternal::TriggerRun(RNode node);
    friend void RDFInternal::ChangeEmptyEntryRange(const RNode &node, std::pair<ULong64_t, ULong64_t> &&newRange);
    friend void RDFInternal::ChangeSpec(const RNode &node, ROOT::RDF::Experimental::RDatasetSpec &&spec);
-
+   friend std::string ROOT::Internal::RDF::GetDataSourceLabel(const RNode &node);
    std::shared_ptr<Proxied> fProxiedPtr; ///< Smart pointer to the graph node encapsulated by this RInterface.
 
 public:

tree/dataframe/src/RInterface.cxx

@@ -39,3 +39,14 @@ void ROOT::Internal::RDF::TriggerRun(ROOT::RDF::RNode node)
 {
    node.fLoopManager->Run();
 }
+
+std::string ROOT::Internal::RDF::GetDataSourceLabel(const ROOT::RDF::RNode &node)
+{
+   if (node.fLoopManager->GetTree()) {
+      return "TTreeDS";
+   } else if (node.fDataSource) {
+      return node.fDataSource->GetLabel();
+   } else {
+      return "EmptyDS";
+   }
+}