flow the arcs through actors

Link: https://github.com/GeneAssembly/biosal/issues/510
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
build distributed assembly graph

Link: https://github.com/GeneAssembly/biosal/issues/510
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
add number of kmers and arcs and number of kmer observations

Link: https://github.com/GeneAssembly/biosal/issues/523
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>