genomics: change the minimum coverage to 1

With 2:

[boisvert@bigmem biosal]$ grep '>' output-c2/unitigs.fasta|awk '{print $2}'|sed 's/length=//g'|sort -r -n|head
23427
23427
23427
23427
21321
21321
21321
21321
21321
21321

With 1:

[boisvert@bigmem biosal]$ grep '>' output/unitigs.fasta|awk '{print $2}'|sed 's/length=//g'|sort -r -n|head
32675
32675
32675
32675
32675
27847
27847
27847
27847
27847

This should yield better contiguity in metagenome assemblies.

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