Add const mesh getter to VolumeMeshObject

ObjectTypes/VolumeMeshObject/VolumeMeshObject.hh

@@ -122,6 +122,7 @@ public:
   public:
     /// return a pointer to the mesh
     MeshT* mesh();
+    const MeshT* mesh() const;
 
     /** return a full copy of this object ( All scenegraph nodes will be created )
      *  but the object will not be a part of the object tree.

ObjectTypes/VolumeMeshObject/VolumeMeshObjectT.cc

@@ -216,6 +216,11 @@ MeshT* VolumeMeshObject<MeshT>::mesh() {
     return mesh_.get();
 }
 
+template<class MeshT>
+const MeshT* VolumeMeshObject<MeshT>::mesh() const {
+    return mesh_.get();
+}
+
 /** Updates the visualization of the object. Calls VolumeMeshObject::updateGeometry,
  *  VolumeMeshObject::updateTopology, VolumeMeshObject::updateSelection and
  *  VolumeMeshObject::updateModelingRegions.