Add steering dependant turning moment coefficient

[ufechner7]

The turn rate of a kite depends on the side force, but also on the steering induced deformation of the side surfaces. Implement the second effect to be able to achieve the correct value of the turn rate law coefficient without the need for unrealistically high side forces.

[ufechner7]

Done:

    f_d = 0.5 * rho * s.set.area * norm(va_xz1)^2 * (s.set.cmq * s.pitch_rate * s.set.cord_length) * z
    f_s = 0.5 * rho * s.set.area * (0.5*(norm(va_xy3)+norm(va_xy4)))^2 * (s.set.smc * rel_steering * s.ks) * x
    s.forces[s.set.segments + 2] .+= f_d
    s.forces[s.set.segments + 4] .+= (L3 + D3 -0.5*f_d + 0.5*f_s)
    s.forces[s.set.segments + 5] .+= (L4 + D4 -0.5*f_d - 0.5*f_s)