Better Backbone implementation for Segments

[patmo141]

The backbone is a path which is cut along the surface of the original form. It is generated using the same cutting algo as each of the individual loops in a segment. The loops can be thought of as disks between vertebra, separating the backbone into individual parts. It passes through the 0th vert in closed forms (rings).

1. Improve updating
   Currently, the backbone is regenerated along the whole segement in many instances where some intelligence could allow for just the appropriate "vertebra" to be updated.
2. Implement new cutting algo
   Sometimes the cutting algo fails to generate a vertebra between disks for the same reason we were getting errors in the loop cuttting
3. Used for interpolation between segments. Allows for a quick and easy intersection test with any cut plane to determine new insertion of
4. Use to preserve alignment and shift