BUGFIX: PreviousTickTime should be initialized to the instantiation time

As reported by Jim Kier, TickedVehicle.PreviousTickTime should actually
be initialized with the time that the vehicle was instantiated, not zero.
Otherwise the first force calculation that relies on it would yield
erroneous values.
BUGFIX: SteerForPathSimplified was aiming too far ahead

For some historical reason SteerForPathSimplified was seeking a point
that was beyond its estimated future position.  We could add this
separately as a parameter, but it makes no sense as the default
behaviour.
BUGFIX: SteerForPathSimplified should check for zero-length path
SteerForPathSimplified.PredictionTime should not be negative
Improved Pathway code comments
Vector3Pathway.AddPoint should not be public

We'll have a single point for modifying the pathway, it will be the call
to Initialize.
New SplinePathway
General code clean-up
BREAKING CHANGE: Pruned cyclic Vector3Pathways, further clean up

- Cyclic had never been properly implemented, and they're just as doable
  by having an event handler for the arrival even of the path steering
  behavior.
- Removed redundant Points property, which was there just because on
  cyclic paths the point list was different than the original path list.
Better IPathway documentation
BREAKING CHANGE: Removed redundant Pathway

Likely any pathway can be done as a subclass of Vector3Pathway, making
its abstract base class redundant. Removed.  Keeping IPathway.
Marking sample behaviors as obsolete, chances are I'll remove them
UpdateOrientationVelocity is now protected

Only subclasses should access it, so there's no reason for it to be public
Renamed OpenSteerUtility methods for consistency
Vector3Pathway.DrawGizmos should be overridable by subclasses
Removing old sample path behaviors, keeping them as gists

https://gist.github.com/ricardojmendez/88488a8550ea62bfa119
https://gist.github.com/ricardojmendez/f4fff18b34faa0ce17bd
Revert "SteerForPathSimplified was aiming too far ahead"

Oops. That line was *not* aiming too far ahead - I should read my own
comments before doing hasty changes.

This reverts commit ccc702cb67879c69da869726aca4c00fe3c90bd8.
Vector3Pathway, SplinePathway improvements and bugfixes

- Changed calculation for Catmull-Rom points.
- Removed GoKit dependency for debug.
- Moved path data precalculation to its own method.
- Reorganized Vector3Pathway so that properties are grouped.
- Handled case where we're aiming for beyond the path's length.
Improved vehicle code documentation
Further code clean-up

- Changed float comparisons to use Mathf.Approximately
- Removed unnecessary assignments
- Renamed properties for consistency
- Fixed comments
- Added TODOs