Pathway / IPathway clean-up

- Moved Radius to IPathway. It can always be zero if it's not needed.
- Made abstract Pathway methods that were unnecessarily virtual, to
  reduce clutter.
New sample post-processing behavior: SteerForSpeedFluctuation
BUGFIX: SteerForMinimumSpeed should default to the desired velocity

Defaulting to Vector3.zero works when it's the only post-processing
behavior being applied, but otherwise it would be averaged with the
other behaviors and would be slowing them down.