GraphWalker: better tracking path age

Major change: Change how we traverse with paths
Change behaviour of GraphWalker to remember age of paths by when they were picked
up rather than how many junctions they have been used for. This seems to fix
the issues with the exp_abc simulations.

Minor:
ctx_exp_abc.c: add option to print failed contigs
ctx_supernodes.c: More changes to get ready to read in paths to print annotations
scripts/cortex_to_graphviz.pl: Bug fix when loading paths file
Fix bugs in reporting assembled contig stats