cola layout temporarily NaNs a lot of node positions

This is a very old bug, maybe going back to v0.0.

When nodes are added in cola layout only, they start at (NaN, NaN), which appears as (0,0). Much spew ensues in the console.

This happens in explore.html (causing lousy transitions) but not in random.html, so it probably has to do with whether `showLayoutSteps` is enabled, or some other option that is cola-specific.