thorium: enable routing in messages

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
thorium: add router with a polytope virtual topology to begin with

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
tests: add unit test for polytope topology and router

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