simplify file names

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
simplify the spawning procedure

Also, this commit only puts receive handlers in vtable for simplicity.
In fact, vtable are not used anymore since there is only one
handler per actor.

Link: https://github.com/GeneAssembly/biosal/issues/15
Link: https://github.com/GeneAssembly/biosal/issues/16
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>