don't exit if there are messages to send but no actors

Link: https://github.com/GeneAssembly/biosal/issues/87
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
implement remote spawning (BSAL_ACTOR_SPAWN)

A significant part of the code for this feature
was pushed in 6d1ed644a. But while working on
the BSAL_ACTOR_SPAWN feature, I discovered a bug
(a race) which I had to track down.

Link: https://github.com/GeneAssembly/biosal/issues/84
Link: https://github.com/sebhtml/biosal/commit/6d1ed644a81d47cf39d6a14affce5ae49728c50b
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
rename bsal_actor_vtable to bsal_script

Link: https://github.com/GeneAssembly/biosal/issues/85
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
change input_vtable to input_operations

Link: https://github.com/GeneAssembly/biosal/issues/88
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
complete node API

Link: https://github.com/GeneAssembly/biosal/issues/86
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>