AC: struct refactor

[AlexS12]

Current situation
Currently, each aircraft implements its own structure as it is a mean to dispatch the adequate method for functions like: get_arp, get_empty_cg, get_fuel_tanks... This is nonsense because:

• some of this methods need to be called before or during the aircraft object construction (see ugly workarounds in: C310() and F16())
• even there is a struct for each aircraft, they share the same attributes
• some ac variables are stored in methods to benefit from constant propagation (http://www.stochasticlifestyle.com/type-dispatch-design-post-object-oriented-programming-julia/) but this prevents from making variations in these parameters (ie. empty cg position is changed in many F16 validation examples)

Objective

• Create a unique struct to be shared by every aircraft
• Separate aircraft parameters in a different struct that will be part of the aircraft object.
  • These parameters struct can be specific to each aircraft until a good generalization is found. However, a set of general methods to access common attributes must be provided.
  • For each aircraft a default configuration will be provided.

Solution proposal

abstract type Parameters end

get_name(p) = p.name
get_wing_area(p) = p.wing_area
get_wing_span(p) = p.b
get_chord(p) = p.mac
get_arp(p) = p.arp

struct Aircraft
    parameters::T where T<:Parameters
    mass_props::RigidSolid
    pfm::PointForcesMoments
    aerodynamics::T where T<:Aerodynamics
    propulsion::Propulsion
end

F16(params, mass_props, pfm, aero, prop) = Aircraft(params, mass_props, pfm, aero, prop)

function F16()
    # default initialization of params, mass_props, pfm, aero, prop
    # ...
    Aircraft(params, mass_props, pfm, aero, prop)
end

Concerns
This will not benefit from constant propagation anymore.