New Train Interrupt Condition - Pending Upgrade

[_sjy]

Hi,

The recent FFF indicated that robots know how to upgrade trains "in-place", as it were.

If you have a global robot network, upgrading a large train fleet is therefore straightforward.

If you don't have a global train network, you'd like trains with pending upgrades to come visit the mall at some point, so they get upgraded.

It'd be nice to have a "pending upgrade" interrupt condition

If we had that, we could have a high priority interrupt that checks for empty cargo AND has pending upgrade AND upgrade bay has available train limit, add upgrade bay as a destination.

Thanks for all the work on 2.1!