improve selection criterion

choice_coverage < 2 * current_coverage
AND
other_choice_coverage < current_coverage / edge_count

Link: https://github.com/sebhtml/ray/blob/master/code/SeedingData/SeedWorker.cpp
Link: https://github.com/GeneAssembly/biosal/issues/100
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>