Evaluate a Bspline or its derivatives.
Given the knots and coefficients of a Bspline representation, evaluate the value of the smoothing polynomial and its derivatives. This is a wrapper around the FORTRAN routines splev and splder of FITPACK.
Parameters :  x : array_like
tck : tuple
der : int
ext : int


Returns :  y : ndarray or list of ndarrays

References
[R14]  C. de Boor, “On calculating with bsplines”, J. Approximation Theory, 6, p.5062, 1972. 
[R15]  M.G. Cox, “The numerical evaluation of bsplines”, J. Inst. Maths Applics, 10, p.134149, 1972. 
[R16]  P. Dierckx, “Curve and surface fitting with splines”, Monographs on Numerical Analysis, Oxford University Press, 1993. 