Allow duplicate id's in input_nodes and output_nodes

For @svensson it would be convenient to allow multiple internal nodes to be referred to be a single input or output id. When ewokscore loads the graph, it will make multiple connections in case duplicates occur.

The current alternative would be to link the multiple internal nodes to another internal node (e.g. with task_type ppfport). This will cause overhead during execution because the scheduler will have to execute that node.