set unitig flag for vertices

Link: https://github.com/GeneAssembly/biosal/issues/664
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
unitig: spawn walkers

Link: https://github.com/GeneAssembly/biosal/issues/679
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
restore the walking code

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
use unitig markers

Link: https://github.com/GeneAssembly/biosal/issues/679
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>