add const MaterialNode getter

common/BaseObjectData.cc

@@ -270,6 +270,10 @@ MaterialNode* BaseObjectData::materialNode() {
   return materialNode_;
 }
 
+const MaterialNode* BaseObjectData::materialNode() const {
+    return materialNode_;
+}
+
 BoundingBoxNode* BaseObjectData::boundingBoxNode() {
   return boundingBoxNode_;
 }

common/BaseObjectData.hh

@@ -193,7 +193,9 @@ class DLLEXPORT BaseObjectData : public BaseObject
 
     /// get a pointer to the materialnode
     MaterialNode* materialNode();
-
+    
+    const MaterialNode* BaseObjectData::materialNode() const;
+    
     /// get a pointer to the bounding box node
     BoundingBoxNode* boundingBoxNode();