Move parameter blocks vector outside loop

This reduces the amount of allocations required, since the same vector
and its previously allocated memory is reused among all the graph
constraints when the ceres::Problem is created.

fuse_graphs/src/hash_graph.cpp

@@ -464,11 +464,12 @@ void HashGraph::createProblem(ceres::Problem& problem) const
     }
   }
   // Add the constraints
+  std::vector<double*> parameter_blocks;
   for (auto& uuid__constraint : constraints_)
   {
     fuse_core::Constraint& constraint = *(uuid__constraint.second);
     // We need the memory address of each variable value referenced by this constraint
-    std::vector<double*> parameter_blocks;
+    parameter_blocks.clear();
     parameter_blocks.reserve(constraint.variables().size());
     for (const auto& uuid : constraint.variables())
     {