WIP Initial rewrite of ethernet IO code.

 - Redefines some of the API for IO builders
 - Creates new SDPRx/Tx elements

TODO:
 - Build a map between placement and node output
 - Provide the API for setting node output and retrieving node input
 - Provide threading

LONGER TERM:
 - Provide slicing on the Tx element
 - Use the new SpiNNaker IO packet formats for SDP?  I'm not sure this
   gets us anything other than more overhead, the packet convention here
   is sufficiently simple for others to implement...