Need better handling of cars switched in yard mid-session

[bowdidge]

Layouts with yards often want cars brought in by one train to be taken out by another train in the same session. SwitchList currently doesn't handle this well.

Potential solutions:

• define schedules so we know what trains will come in early enough for their cars to be switched to later trains.
• manually allow cars to be reassigned at some point during the session. (This can be done by switching to a different tab, then back to overview). This could be implemented as a "re-assign cars" button.