smooth_1d_tree too aggressive

[tawhai]

objective of the subroutine is to avoid having child branches out of plane with parent. method currently just sets the common node between parent and child branches to be mid-way between start of parent and end of children. causes the parent branch to become shorter than it should. change this to calculate average of the start and end nodes, adjust common node so that parent length is the same and parent branch is in the direction of the average calculation.