extrapolate forbidden with interp1, spline

[Eric Chassande-Mottin]

hi,

I just wanted to mention that  the extrapolation
(of a point outside the support of the input data set)
is *not* allowed using interp1() with opt "spline" in GNU
Octave, while it is in the corporate world.

see:

octave:1> interp1([1 2 3 4],[1 2 3 4],-1,'spline')
ans = NA

interp1 is the user-defined function from the file
/usr/share/octave/2.9.14/m/general/interp1.m

>> interp1([1 2 3 4],[1 2 3 4],-1,'spline')

ans =

    -1

perhaps the way to solve this incompatibility
is to invite the use of the 'extrap' opt with a warning

octave:22> interp1([1 2 3 4],[1 2 3 4],-1,'extrap')
ans = -1

cheers, eric.