AI Update:

AI Update:
 - Ground network slow-down finally works as expected
   (although occasionally causing a traffic jam)
 - Hold position instruction now really sets speed to zero, in addition
   it actually works now for crossing and two-way traffic
 - Attempt to limit execution time of ground network trace algorithm
   to make performance acceptable at high-density networks
 - Removed remaining terminal messages
 - Various minor tweaks and clean-ups