Add fancy length abbreviations to visualization default axis labels

[fedarko]

e.g. replacing "12,345,678 bp" with "12.3 Mbp".

Something of note is that we use nt instead of bp, which is fine (and arguably better as a default "unit"), but I haven't seen anyone say knt / Mnt / etc. in the literature. So maybe it would be worth switching the default axis labels to say bp, while we're at it ...? I don't think it really makes a difference.

For details on the units thing:

• https://www.seqanswers.com/forum/bioinformatics/bioinformatics-aa/59237-b-p-or-nt-which-is-correct
• https://forum.qiime2.org/t/unit-of-measurement-for-sequence-length-unit-base-pairs-versus-unit-nucleotides/22684
• https://www.reddit.com/r/labrats/comments/1dtnx4w/are_base_pairs_bp_and_nucleotides_nt_the_same/