nderiv_fornberg( k, xPts, u)

Computes a derivative of arbitrary order on a vector u on an uneven grid.

This routine computes the k^th derivative of a vector (u) defined on a set of points (xPts) using Fornberg's algorithm.
Inputs:
k, the order of the derivative you wish to compute.
xPts, the set of points on which u is evaluated.
u, the array of values u(x) which are evaluated on xPts (u(i) := u(xPts(i))).

Requires/Assumes:
* xPts should be sorted in ascending order
* k < length(xPts)

Note that this submission requires the fdcoeffF.m function which is available here:

http://faculty.washington.edu/rjl/fdmbook/matlab/fdcoeffF.m

This routine, in turn, is based on the program "weights" in

B. Fornberg, "Calculation of weights in finite difference formulas", SIAM Review 40 (1998), pp. 685-691.