use nominal case to aid in derivative of angular refraction

[fgnievinski]

[fgnievinski]

implemented:

    if opt.der_aided
        [de0, der0] = get_bending_nominal (e, N);  % nominal bending and its rate
        rde = de - de0;  % residual bending
        rder = gradient_all(rde, e, [], opt.numerical_noend);  % residual bending rate
        der = der0 + rder;  % reconstructed total bending rate
        der(der==0) = NaN;
          %fpk e [de de0 rde]  % DEBUG
          %fpk e [der der0 rder]  % DEBUG
    end

[fgnievinski]

results posted at:
#12