[Ifeffit] Spline clamps

[Matt Newville]

Hi Scott, 

The clamp uses the last data points, not the points near kmax (and
similarly it uses the data points at above k=0, not k=kmin).  I'm
not sure if this is a bug (that is, it's not obvious what 'right' is
here), but it's probably not the behavior you were hoping for ;).

It's probably safe to assume that the clamps should persuade chi to
be close to 0 at kmin and kmax (that is, this seems as safe as
assuming chi should be near 0 at k=0 and k=last_data_point), so I
don't see much reason to keep it like it is.  I'll (try to) make the
clamps use the points near kmax (and kmin) for 1.2.5

--Matt