Objective

• Fixes Rounding errors can accumulate and cause nodes to expand each time a layout is recomputed #501

Notes

• This fix is a bit hacky. I think ideally we'd want to differentiate between rounded and unrounded values in the public API. But the advantage of this implementation is that it's backwards compatible and can be released as a patch to the 0.3.x series. We can of course always improve upon this in the 0.4 release.
• It requires us to store both rounded and unrounded values, adding (I believe) 20 bytes per node. This is not ideal, but it seems like the simplest way to fix the issue. Perhaps in future we could improve upon this on future too.

Feedback wanted

General code review