Support loading old models in ML module

- added test for loading legacy files
- added version to new written models
- fixed loading of several fields in some models
- added generation of new fields from old data

modules/ml/src/rtrees.cpp

@@ -346,7 +346,7 @@ public:
         oobError = (double)fn["oob_error"];
         int ntrees = (int)fn["ntrees"];
 
-        fn["var_importance"] >> varImportance;
+        readVectorOrMat(fn["var_importance"], varImportance);
 
         readParams(fn);