Iterator for calculating acceleleration needs to avoid current Particle

[weka511]

This test is a kludge...
Node::Visitor::Status BarnesHutVisitor::visit(Node * node) {
...
switch (node->getStatus()) {
...
default:
std::cout<<FILE <<", " <<LINE<< std::endl;
dsq=sqr(x-_x) + sqr(y-_y) + sqr(z-_z);
if (dsq>0)_accumulate_acceleration(m,x,y,z,dsq); //FIXME
return Node::Visitor::Status::Continue;
}
}

[weka511]

Either put an id on each Particle, or replace iterator with a for int i loop and pass i in.