sears_lift_sin_gust
Returns the lift coefficient for a sinusoidal gust (see set_gust.sin) as the imaginary part of the CL complex function defined below. The input gust must be the imaginary part of
\[wgust = w0*\exp(1.0j*C*(Ux*S.time[tt] - xcoord) )\]
with:
\[C=2\pi/L\]
and xcoord=0
at the aerofoil half-chord.