change workers-per-node to threads-per-node

Link: https://github.com/GeneAssembly/biosal/issues/46
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
document "make run" and "make test" in readme

Link: https://github.com/GeneAssembly/biosal/issues/50
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
check if fifo is empty in bsal_worker_thread_pull_message

Link: https://github.com/GeneAssembly/biosal/issues/49
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
add between node and within node parallelism in readme

Link: https://github.com/GeneAssembly/biosal/issues/51
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
add provided thread level

Link: https://github.com/GeneAssembly/biosal/issues/47
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
add communication threads in a node

This feature needs MPI_THREAD_MULTIPLE which is not available
in most Linux distribution mpich packages.

Suggested-by: Fangfang Xia <fangfang@anl.gov>
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>