network_base: Function for checking connection

Added a function `connection_exists` that checks if agents with indices
`i_idx` and `j_idx` are connected or not.

graph_lib/include/network_base.hpp

@@ -112,6 +112,19 @@ template <typename WeightType = double> class NetworkBase {
     return weight_list[agent_idx][index_neighbour];
   }
 
+  /*
+  Checks if a connection exists between two agents i_idx and j_idx
+  */
+  bool connection_exists(size_t i_idx, size_t j_idx) const {
+    auto i_neighbours = get_neighbours(i_idx);
+    if (std::find(i_neighbours.begin(), i_neighbours.end(), j_idx) ==
+        std::end(i_neighbours)) {
+      return false;
+    } else {
+      return true;
+    }
+  }
+
   /*
   Clears the network
   */