Node encapsulated length and offset inside getters and setters.

Needed for data encapsulation.
Node added method get_length() to avoid calculating lengths.

Bone length is not likely to change so better to have catched its size
Fixed out of range error on EnhancedIKsolver.

When solve_chain_keep_next_bone_pos would fail to put the next bone back to
where it was previous_rotations would be overwritten and with a smaller size.
Fix error on add bone to chain in IKSolver.

The bone offset was not being changed to KDL coordinate system.
It used to work because luckily the x axis was the same but now
we have x, y, z bone offsets.