fix vtk writer usage in partition

Author: Chrismarsh

src/preprocessing/partition/main.cpp

@@ -110,7 +110,11 @@ class preprocessingTriangulation : public triangulation
         const bool write_ghost = _write_ghost_neighbors_to_vtu;
         _write_ghost_neighbors_to_vtu = false;
 
-        vtk_writer writer(this, true, true);
+        // Wrap `this` in a non-owning shared_ptr so we can pass the base mesh type.
+        // via a no-op deleter
+        auto self = boost::shared_ptr<preprocessingTriangulation>(this, [](preprocessingTriangulation*) {});
+        auto base = boost::static_pointer_cast<triangulation>(self);
+        vtk_writer writer(base, true, true);
         writer.set_write_ghost_neighbors(_write_ghost_neighbors_to_vtu);
         writer.update_data(output_variables);
         writer.write_vtu(file_name);