Loadable Function: val = spwinterp (alfa, wing, pol, deriv)

spanwise inerpolation of polar data. alfa is a vector of local angles of attack at z(j), and cf and idx determine the relative distribution of z w.r.t zp (the spanwise polar positions) as follows: if idx(i) <= j < idx(i+1) then z(j) = (1-cf(j))*zp(i) + cf(j)*zp(i+1), with 0 <= cf(j) <= 1. the inerpolation at z(j) is obtained by weighting the interpolations at zp(i), zp(i+1) by 1-cf(j), cf(j).