make unitig generation deterministic

Test 1

scripts/assembly_debugging/check-signatures.sh 1.fasta

[boisvert@bigmem biosal]$ sha1sum uniq-names
3623792a418a7352db383232fd21a5e32ef7ba50  uniq-names
[boisvert@bigmem biosal]$ wc -l uniq-names
1469 uniq-names
[boisvert@bigmem biosal]$ ls -lh 1.fasta
-rw-rw-r--. 1 boisvert boisvert 8.9M Sep 18 12:59 1.fasta

Test 2

scripts/assembly_debugging/check-signatures.sh 1.fasta

[boisvert@bigmem biosal]$ sha1sum uniq-names
3623792a418a7352db383232fd21a5e32ef7ba50  uniq-names
[boisvert@bigmem biosal]$ wc -l uniq-names
1469 uniq-names
[boisvert@bigmem biosal]$ ls -lh 1.fasta
-rw-rw-r--. 1 boisvert boisvert 8.9M Sep 18 13:08 1.fasta

Link: https://github.com/GeneAssembly/biosal/issues/683
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>