Add node to computation graph by ID

Added ability to create nodes with arbitrary IDs. This should only be used if necessary, preferring graph_add_node over graph_add_node_id, because it does not use hashing, and must allocate an array up to the size of the ID you requested.

The ability to add nodes by ID is necessary for the ground station to use the existing graph infrastructure for showing nodes. We could have left the requirement such that after all nodes are added, they must be consecutive, because that is what the quadcopter will return. I am not a fan of this, though, because it seems like something that will bite teams in the future if they don't know about that unexpected behavior.

quad/src/computation_graph/computation_graph.c

@@ -186,7 +186,8 @@ double graph_get_output(const struct computation_graph *graph, int node_id, int
 /*
  * Assumptions: The node passed in is a valid ID (should be checked before passing)
  * and all node sources are either valid node-output pairs, or the source node ID == -1
- * This function does not check those assumptions for speed
+ * These constraints should be satisfied by using the graph_set_source function, so long as
+ * a valid node ID is passed in to the first call of this function
 */
 void graph_compute_node_rec(struct computation_graph *graph, int node_id, int depth) {
     if (depth >= GRAPH_MAX_DEPTH) {