Add abstract winch model

[ufechner7]

Current definition, but not yet in this package:

abstract type AbstractWinchModel end
const AWM = AbstractWinchModel

"""
    mutable struct AsyncMachine

Model of a winch with an async generator and a gearbox.
"""
@with_kw mutable struct AsyncMachine <: AbstractWinchModel @deftype Float64
    set::Settings
    "nominal motor voltage"
    u_nom = 400.0/sqrt(3)
    "rated synchronous motor speed [rad/s]"
    omega_sn = 1500 / 60 * 2π
    "rated nominal motor speed [rad/s]"
    omega_mn = 1460 / 60 * 2π
    "rated torque at nominal motor speed [Nm]"
    tau_n = 121
    "Rated maximum torque for synchronous motor speed [Nm]"
    tau_b = 363
    "minimal speed of the winch in m/s. If v_set is lower the brake is activated."
    v_min = 0.2
    "linear acceleration of the brake [m/s²]"
    brake_acc = -25.0
    "if the brake of the winch is activated"
    brake::Bool = true;
    "last set speed"
    last_set_speed = 0.0;
end

"""
    mutable struct TorqueControlledMachine

Model of a winch with an torque controlled generator and a gearbox.
"""
@with_kw mutable struct TorqueControlledMachine <: AbstractWinchModel @deftype Float64
    set::Settings
    "winch speed controller"
    wcs::WinchSpeedController
    "minimal speed of the winch in m/s. If v_set is lower the brake is activated."
    v_min = 0.2
    "linear acceleration of the brake [m/s²]"
    brake_acc = -25.0
    "if the brake of the winch is activated"
    brake::Bool = true;
    "last set speed"
    last_set_speed = 0.0;
end

And in the pull request system-structure:

"""
    mutable struct Winch

A set of tethers (or a single tether) connected to a winch mechanism.

$(TYPEDFIELDS)
"""julia
mutable struct Winch
    const idx::Int16
    const tether_idxs::Vector{Int16}
    tether_len::Union{SimFloat, Nothing}
    tether_vel::SimFloat
    tether_acc::SimFloat
    set_value::SimFloat
    brake::Bool
    const force::KVec3
    gear_ratio::SimFloat
    drum_radius::SimFloat
    f_coulomb::SimFloat
    c_vf::SimFloat
    inertia_total::SimFloat
    friction::SimFloat
end

We should define a base type with a common set of fields and/or methods.