User-defined options for AircraftModel

[kanekosh]

It is not straightforward to set options for a user-defined aircraft model (e.g. B738AirplaneModel), although it is possible.

This is because an aircraft class is not instantiated by user, but it is passed to OpenConcept's mission analysis group as a class (not as an instance) and is instantiated internally by OpenConcept. Therefore, the user has to use model_options at a higher-level group to pass down the options to aircraft class.

This should be documented somewhere. Also I think we can get rid of the global variables in this example by using model_options.