Add option for multiple kites

[1-Bart-1]

Currently, the MTK model only has one kite. It should be possible to add multiple kites.

struct PointMassSystem
    name::String
    points::Vector{Point}
    groups::Vector{KitePointGroup}
    segments::Vector{Segment}
    pulleys::Vector{Pulley}
    tethers::Vector{Tether}
    winches::Vector{Winch}
    kites::Vector{Kite}
end

@with_kw struct Kite
    mass::SimFloat = 1.0
    orient::KVec4 = zeros(KVec4)
    angular_vel::KVec3 = zeros(KVec3)
    pos::KVec3 = zeros(KVec3)
    vel::KVec3 = zeros(KVec3)
end