Matlab compatiable spline/csape

[David Bateman]

Trying to port some of the matlab core functions to octave core, I was 
looking at the spline, mkpp and ppval functions last night. Firstly, as 
csape is not a matlab core function, it should be ported to octave core, 
and so some portion of csape need to be cut and paste into spline.m. 
Also when y is two elements longer than x, a complete spline should be 
used, and I made this change. I also replaced things like

     c(2:n - 1,:) = trisolve(ldg,dg,udg,g);

with

     c(2:n - 1,:) = 
spdiags([[ldg(:);0],dg,[0;udg(:)]],[-1,0,1],n-2,n-2) \ g;

as the octave core sparse solver uses the lapack tridiagonal solvers if 
it can. However, I have one compatibility issue in the case where y is a 
matrix or an NDArray. Currently the octave-forge spline function allows  
an n-by-d matrix to be passed for y where n is the length of x, and the 
matrix y is treated column by column, whereas the matlab function allows 
an NDArray of the form d1-by-d2-by- ... -dk-by-n which is exactly the 
reverse. I can probably support both in the majority of cases, however 
ppval and mkpp would need to be modified to store and/or determine which 
is used. Alternatively, just using the matlab syntax would keep mkpp and 
ppval pretty much identical to what they currently are (with minor mods 
for NDArrays). Should I attempt to support both formats for NDArrays, or 
should I just drop the current octave-forge way of treating Matrixs y?