The following improvements are made here:
- merged and streamlined the two IsCanceling checks in Tick
into one that covers both isCloseEnough cases
- isCloseEnough now only checks distance to host
if host is still valid and otherwise returns 'false'
- called transports are now also cancelled when host becomes
invalid, not only if the activity is cancelled via order
- aircraft now always take off if the host becomes invalid
- ground actors only try to leave if host is still existing
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