add acquaintance concept in readme

Link: https://github.com/GeneAssembly/biosal/issues/142
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
use vector to store actors in node

Link: https://github.com/GeneAssembly/biosal/issues/144
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
clone an actor

Link: https://github.com/GeneAssembly/biosal/issues/140
Link: https://github.com/GeneAssembly/biosal/issues/137
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>