[OctDev] added features to butter.m

[Doug S. <da...@sy...>]

I have done a major update to butter.m
   It now can make Laplace space filters and produce zpg and ss outputs.
The syntax is the same as Matlab's butter.m but the results are slightly 
different. I did not refer to matlab's code just compared results.

## [...] = butter(...,'s')
##     return a Laplace space filter, W can be larger than 1.
##
## [a,b,c,d] = butter(...)
##  return  state-space matrices

    The ss results are different because ss info can be presented 
differently. The results I get are compatible with ss2sys so all is ok.
For the zpg results what I get are compatible within octave and I 
believe are correct.
    I have named my ver. butter1.m  but hope that after it is approved 
that it will replace butter.m. I have also included two test files to 
help you test what I have done.

I hope that this meets your approval.
My next proj. is to do the same to cheby1.m and cheby2.m., then 
wavread.m and wavewrite.m
My system is a w2k athalon with Andy Adler's binaries (GNU Octave, 
version 2.1.42 (i686-pc-cygwin))
Doug Stewart