add documentation for the storage engine

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
load and dump graph paths

Link: https://github.com/sebhtml/ray/issues/188
Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
do a list of symmetric relations

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>
add a barrier to get deterministic results

Signed-off-by: Sébastien Boisvert <sebastien.boisvert.3@ulaval.ca>