assembly_vertex: improve boundary checks

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
genomics: add flags in vertex neighborhood

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
graph_store: add action ACTION_ASSEMBLY_GET_VERTEX_AND_SET_VISITOR_FLAG

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