- The SCAN block no longer relies on invisible objects.  Instead, it

  checks your position against the anomalyPosition variable on the
  blackboard
- the writeToBlackboard action can now set a specific value.
- renamed the clearBlackboard action to clearBlackboardValue, so that
  you won't think that it clears the whole blackboard.
- updated the position of the "supplies" that the minirover drops,
  and the rover now appears to pick them up.
- added in the character text along the first leg of the rover tracks.
- lowered the volume of the engine/movement sound a bit, because it
  was drowning out the VO.
- lowered the volume of the level 2 music a bit, but the real solution
  will be to replace it.
- fixed the gratuitous error message in hudInstructions.js.
- finished expunging the old "object sensor."  We're using the "track
  sensor."
- Made it so that level 3c will only say "this one is a bit tricky" if
  you start out facing north (because otherwise it's not).

@AmbientOSX @BrettASwift 
I'll fix in a future PR.  Is it better do delete the entry, or just set the entry to undefined?  Which is more memory friendly?