visitor: improve data locality

Link: https://github.com/GeneAssembly/biosal/issues/836
Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
unitig_visitor: set CONFIG_VISITOR_LOCALITY_WIDTH to 8192

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
unitig_visitor: align step values

Signed-off-by: Sébastien Boisvert <boisvert@anl.gov>
unitig_visitor: move locality logic in biosal_unitig_visitor_set_locality_kmer

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