Computes the second-order central difference approximation to the first-order space derivative with second-order biased approximation for boundary points.
By using the Newton-Gregory backward polynomial we obtain
For second-order central difference approximation, we need to fit the polynomial through the three points
xi-1, xi, xi+1. This means to write the polynomial for example around the point xi+1,
drop the higher-oder terms and set s = -1 to obtain
For the boundary point x1 we use biased formula and obtain
By using the same idea we obtain a biased formula for the boundary point xn
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