Fix neg number bug in roundToSignificantFigures

And make it a method on View.
Move rounding changes tests into correct place

And remove one duplicate.

And tidy whitespace on an object, remove unused thresholds (they're
duplicated below).
Change rounding thresholds from 500 to 1000

For all orders of magnitude.

I think there's an opportunity for some refactoring here.
Merge pull request #119 from alphagov/54184200-change-rounding-thresholds

Change rounding thresholds