minimize phi(alpha) using a cubic model - linesearch with step length selection

PhiCubicModel.m

    M1 = [alpha_prev^2, -alpha_cur^2; -alpha_prev^3, alpha_cur^3];
    M2 = [phi_alpha - phi_0 - phiP_0*alpha_cur; phi_prev - phi_0 - phiP_0*alpha_prev];
    M = M1 * M2 * 1/(alpha_prev^2 * alpha_cur^2 * (alpha_cur - alpha_prev));
    a = M(1);
    b = M(2);
    alpha = (-b + sqrt(b*b - 3*a*phiP_0))/(3*a);