FTAG GNN inference cleanup

• Improve function and type naming (e.g. "getters" "custom getters" etc)
• Improve type aliasing
• Start work on the V2 onnx metadata version. More to follow, see https://its.cern.ch/jira/browse/AFT-730

PhysicsAnalysis/JetTagging/FlavorTagDiscriminants/Root/ConstituentsLoader.cxx

@@ -143,18 +143,18 @@ namespace FlavorTagDiscriminants {
           trk_type_regexes, trk_sort_regexes, trk_select_regexes,
           flip_sequences, flip_config);
         config.type = ConstituentsType::TRACK;
-        config.output_name = "track_features";
+        config.output_name = "tracks";
       }
-      else if (name.find("flow") != std::string::npos){
+      else if (name.find("flows") != std::string::npos){
         config = get_iparticle_input_config(
           name, input_variables,
           iparticle_type_regexes);
         config.type = ConstituentsType::IPARTICLE;
-        config.output_name = "flow_features";
+        config.output_name = "flows";
       }
       else{
         throw std::runtime_error(
-          "Unknown constituent type: " + name + ". Only tracks and neutrals are supported."
+          "Unknown constituent type: " + name + ". Only tracks and flows are supported."
           );
       }
       return config;