/**
* Evaluate gravitational field on the body.
* The original olden version calls a routine named "walkscan",
* but we use the same name that is in the Barnes code.
*/
public void hackGravity (double rsize, Node root)
{
MathVector pos0 = pos.cloneMathVector ();
HG hg = HG.makeHG (this, pos);
hg = root.walkSubTree (rsize * rsize, hg);
phi = hg.phi0;
newAcc = hg.acc0;
}