Convert a node-centric DBG into an arc-centric one. Currently the input is the BCALM2 fasta format, and the output is an edge list preceded by the number of nodes. The arc-centric dbg is represented in "doubled" form, i.e. a directed graph in which each binode is represented by a pair of nodes and each biarc is represented by a pair of arcs. As an exception, self-complemental arcs are collapsed into a single arc.