Spline interpolation

Syntax: | `vout = INTERPOLATE(x,y,xout,'SPLINE')`

If the `SPLINE` keyword is used the interpolant is calculated by the method of cubic splines under tension. The tension factor corresponds to the "curviness", and must be greater than zero. If it is close to zero, each interpolated function is almost a cubic spline and the resulting curve is quite "loose". If the tension is large, then the resultant is almost linear. The tension used is the current value of `TENSION`, which may be changed with the `SET TENSION` command.

Linear interpolation
Lagrange interpolation