Added default graph constructor

include/gp-bnb/graph.hpp

@@ -8,6 +8,8 @@ using vertex_id = unsigned int;
 // Represents the input graph to be partitioned
 class graph {
 public:    
+
+    graph() = default;
     /** @brief  Initializes the graph to be partitioned
      *  @param  Adjacency lists
      */