Select gates for aircraft in custom schedules#395

When creating a custom schedule, you can define what gate and aircraft can spawn at/land at. This can:

1. Reduce the likelyhood of aircraft clipping into each other (as they’re in specifcally chosen gates). This also means multistands can be included in airports where they aren’t currently
2. Allow for more realistic operations (so as soon as an aircraft leaves a gate, another takes its place), which increases traffic flow and reduces the number of aircraft that don’t spawn