SteerForNeighborGroup and Radar improvements and optimizations

- It doesn't really make sense to have a separate LayersChecked property
  from the one already present on the Radar - if you didn't want to
  consider them neighbors, you shouldn't have scanned for them.
- Radar.OnDetected no longer sends a SteeringEvent, will now just send
  the radar. The extra allocation was unnecessary.
MAJOR CHANGE: Vehicles will always use their own mass for calculations

WARNING: Make note if your vehicles' internal mass if you're using it
at all before upgrading to this version.  Hopefully you have these
things under version control.

Previously we used the Vehicle's Rigidbody's mass as the vehicle's own
unless OverrideRigidbodyMass was set to true.  This added the need for
some unnecessary checks, and moves some settings to outside of the
vehicle itself, when it's best to have things consolidated.

Starting on this commit, I am:
- Removing any difference between mass and internal mass.
- Removing OverrideRigidbodyMass.
- Simplifying the Mass getter/setter.

If an agent wants to alter the attached rigidbody's mass, they should
do so directly and explicitly.
Improvement: No good reason for Vehicle.CanMove to be virtual

This change makes the code easier to grok, no longer has hidden
assignments.

Some vehicles clear their velocity when the CanMove property is set to
false, but this can be done just as well on the Update method - which
TickedVehicle is already doing.  It's not like an assignment is going
to break the performance bank.
Removed redundant normalization for planar ticked vehicles

Unity takes care of forward normalization on assignment, so there's no
good reason to have an extra call in there.
Removing RadarPing, it has been obsolete for a while
Radar detection changes

- Detected colliders are now cached by their instance id, so that we
  can use a SortedDictionary to find them.
- Comparing explicitly against the vehicle instead of leaving that
  up to ignoredObjects.Contains, since that will be the case we will
  encounter more often.
- No need for an explicit Vehicle backing property.
Force should be affected by mass before being clamped to MaxForce

Otherwise we could have a case where a mass that is < 1 ends up
amplifying the force beyond the maximum amount that we can apply.