Core: customizable airplanes

[DennisLent]

Right now we are using a fixed set of airplanes. Perhaps to increase customizability we could allow importing user-created airplanes via the yaml file.

Customization then includes:

• map (airports, locations, sizes, etc)
• initial orders
• airplanes

Then requires rewrite of core & GUI

[DennisLent]

Perhaps also generate specifically airport for only cargo and mixed (makes it more interesting?)