#13 real fft and inverse fft

[Andrea Spinelli]

Added a fft capability to the Vector object.

By now, fft is limited to real functions, but
allows both standard fft and power spectrum
calculation, with an optional Bartlett window.

Inverse fft is possible through the ifft vector
operation.

I also updated the vector.html documentation
and provided a demo, called fft.tcl.

I enclose the tarball with the modified/added files.

I am available for modifications if the patch
does not satisfy some reasonable requirement.

Ah, I left in the code several start/stop markers
with my mail address, spinellia@acm.org. Of
course they are there just for convenience
and should be removed in the production version,
if any.

I tried to contact gah to discuss whether
the patch was acceptable to him, but siliconmetrics.com
considers my e-mails as 'junk mail', so I try
here.

Thank you for developing BLT!
Andrea

[Andrea Spinelli]

Zipped tarball containing the modified files