add an abstraction to fetch a vertex neighborhood

Link: https://github.com/GeneAssembly/biosal/issues/666
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
fetch parents

Link: https://github.com/GeneAssembly/biosal/issues/667
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
fetch parents and children in neighborhood

Link: https://github.com/GeneAssembly/biosal/issues/667
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
modify priority schedulign algorithm

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