spawn one actor for each genome graph file

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
provide the coalescence actor for file actors

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
send payloads to aggregator

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
kill actors to actually end computation

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>