unitig_visitor: don't count the same graph store twice

When a graph store has nothing more to produce, but other graph
stores still do, a graph store may be counted more than once.

This change fixes this bug.

Suggested-by: Élénie Godzaridis <telomerika@gmail.com>
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
genomics: use Doxygen comment for structure

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
graph_store: add some assertions for the iterator

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