change field names in bsal_script

Link: https://github.com/GeneAssembly/biosal/issues/494
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
add the thorium engine

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
simplify the way a program  is written

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>