Create pull rather than push Conveyors

[cwm9]

Instead of selecting which items will be placed into a loader at the loader itself, it would be great if we could remotely control a loader at the destination.  That way we could store items at a central location, then when we want something somewhere we can just ask for it.

This could be added to the current system without breaking existing systems simply by adding a control wire to loaders.  The other end of the control wire can terminate in a material selector building that allows you to remotely set any connected loaders to particular material types.

To make the system even better, if you OR two selector building signals together, it should OR together each selection as if were a bit.  (iron ore OR coal => iron ore, coal).  AND and NOT gates would work similarly.  If you AND a normal RED/GREEN signal with a selector signal, it would pass the selector signal on GREEN or send "no selection" on RED.  (XOR should flip the selection on GREEN and do nothing on RED, OR should send "all selected" on GREEN and pass the selector signal on RED.)

This would allow us to remotely request materials when building.

Getting a pull system to work would require some creative manipulation of signals.  (You'd need a way to track how much material you had of each type so you didn't go overboard, and have some way to return unused materials.  This could all be automated with creativity.)