Round relay bandwidths in bandwidth files

Torflow rounds raw bandwidths to 3 significant figures, and increases 0 bandwidths to 1:

https://gitweb.torproject.org/torflow.git/tree/NetworkScanners/BwAuthority/aggregate.py#n62

Rounding

sbws must round to 3 (or fewer) significant figures, so that consensus diffs are still efficient.

I suggest 2 significant figures, because the largest error is 5% at 1.5*10^n^ (for example: (105-100)/100 = 5%).

Bandwidth authorities already vary from each other by 25-50%: https://trac.torproject.org/projects/tor/ticket/25459#comment:5

And network load varies each day (from what I remember, my guards and exits used to vary by at least 10% every day).

Avoiding Zeroes

I think sbws should also increase 0 bandwidths to 1.

We also have to update the spec, I'll open a child ticket.