Fix duplicated nodes when steered

Fix duplicated nodes when steered

Require steer(f, t) method to set _steered member variable to include f
and t, i.e. f == _steered.front() and t == _steered.back().

This has consequences for collide_steered method as if there is only one
node left, it is already part of the tree and all the other (steered)
nodes collide. In such a case, drop the steered as colliding.