Option to group input and output data blocks near ports.

[tmcphillips]

The data blocks corresponding to workflow inputs and outputs sometimes are positioned far from where the edge connecting the port (small circle outside the workflow box) to the data block enters or exits the workflow box. This makes it harder to see what data each workflow input and output corresponds to.

There should be an option to group input data blocks together as near as possible to the input ports (and output data blocks near the the outputs ports) when graph.portlayout is GROUP. Consider making this the default.