Sankey Plugin, complex path to draw links

As seen in
http://bl.ocks.org/d/4599519/

using a thick line to draw links between blocks can lead to artifacts ( just drag the blocks around) 

this patch uses a set of 4 lines to draw the connection between blocks, see
http://bl.ocks.org/d/4599792/

see also
http://bl.ocks.org/d/4590576/
for the usual example after this patch