Abs weighted average in manyBody barnes-hut coords

[vasturiano]

The quadrant coordinates for the Barnes-Hut approx in forceManyBody are calculated using a weighted average according to the child strengths. However, the current calc is not accounting for negative strengths which may throw off the quad.x/quad.y coords. Furthermore the issue is aggravated when combining positive and negative strengths, which will result in a reduced aggregated strength (due to cancellation), leading to very high values of x and y.

I came across the issue when noticing combined nodes with positive and negative strengths having a much lower force intensity, due to the quadrant centers being much more distant.

This patch abs the weights in the average to fix the issue.

[mbostock]

Thanks!