Fix: reformat long seqs

[joko3ono]

resolve wurmlab/sequenceserver-cloud#415

[djleite]

things look almost ok

however, there needs to be correct rounding up of the final sequence length. See below that:

• 3,999,920 bp should be 4 Mbp
• 799,920 bp should be 800 Kbp

However...

If we want to show a greater level of precision then we can show to at least 1 decimal place for any value that has units kbp, Mbp and Gbp.

ie divide these numbers by 1000 and round to the nearest decimal place

• 19,299 bp = 19.3 kbp
• 19,990 bp = 20.0 kbp

divide these numbers by 1000000 and round to the nearest decimal place

• 199,929,999 bp = 199.9 Mbp
• 199,990,000 bp = 200.0 Mbp

[tadast]

@joko3ono also please make sure you add tests for the rounding so that it's not broken in future