function _quadIndex(point, quadBounds) {
    var xmp = quadBounds.x + quadBounds.width / 2,
        ymp = quadBounds.y + quadBounds.height / 2,
        top = (point.y < ymp),
        left = (point.x < xmp);

    if (top) {
      if (left)
        return 0;
      else
        return 1;
    }
    else {
      if (left)
        return 2;
      else
        return 3;
    }
  }
Get the index of the node containing the point in the quad

@param  {object}  point      A point defined by coordinates (x, y).
@param  {object}  quadBounds Boundaries of the quad (x, y, width, heigth).
@return {integer}            The index of the node containing the point.