For reasons that I can’t recall, the SI-prefix behavior was different for small
numbers (between -1 and 1) than it was for large numbers. This commit enforces
consistent behavior, so that the coefficient is always in the range [1, 1000),
like in engineering notation.

For example, the old d3.format("s") would display 0.01 as "0.01", whereas the
new behavior displays it as "10m".