Cubic Interpolation of a 1-D function.
YI=ICUBIC(Y,XI) returns the value of the 1-D function Y at the
points XI using cubic interpolation. length(YI)=length(XI). XI is
an index into the vector Y. Y is the value of the function
evaluated uniformly on a interval. If Y is a matrix, then
the interpolation is performed for each column of Y.
If Y is of length N then XI must contain values between 1 and N.
The value NaN is returned if this is not the case.
YI = ICUBIC(X,Y,XI) uses the vector X to specify the coordinates
of the underlying interval. X must be equally spaced and
monotonic. XI must lie within the coordinates in X.