don't pack length in kmer_store_pack

Link: https://github.com/GeneAssembly/biosal/issues/249
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
write canonical coverage distribution

Link: https://github.com/GeneAssembly/biosal/issues/250
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
add -Wconversion and -Wextra

Link: https://github.com/GeneAssembly/biosal/issues/248
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>