encode sequence data in 2 bits

before
argonnite.sh real    1m0.869s
scripts/run-argonnite-3.sh  5m24.408s

after
argonnite.sh real    1m51.396s
scripts/run-argonnite-3.sh  10m53.464s

Link: https://github.com/GeneAssembly/biosal/issues/181
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>