Construct an interpolator passing through the specified points
The polynomial passes through all the pairs (xi,yi). One may additionally specify a number of derivatives at each point xi; this is done by repeating the value xi and specifying the derivatives as successive yi values.
Parameters :  xi : arraylike, length N
yi : arraylike, N by R


Examples
To produce a polynomial that is zero at 0 and 1 and has derivative 2 at 0, call
>>> KroghInterpolator([0,0,1],[0,2,0])
This constructs the quadratic 2*X**22*X. The derivative condition is indicated by the repeated zero in the xi array; the corresponding yi values are 0, the function value, and 2, the derivative value.
For another example, given xi, yi, and a derivative ypi for each point, appropriate arrays can be constructed as:
>>> xi_k, yi_k = np.repeat(xi, 2), np.ravel(np.dstack((yi,ypi)))
>>> KroghInterpolator(xi_k, yi_k)
To produce a vectorvalued polynomial, supply a higherdimensional array for yi:
>>> KroghInterpolator([0,1],[[2,3],[4,5]])
This constructs a linear polynomial giving (2,3) at 0 and (4,5) at 1.