[GSLL-devel] added basic fft to my dataframes

[Mirko Vukovic]

In preparation for experiments that start tomorrow, I added today 
real-array fft and power spectrum capabilities to my dataframe code.  It 
uses GSLL's fft functions (search for file fft.lisp).  The classes, generic 
functions, and methods are still a bit rough, and will be refined.

You can read more about it: 
https://github.com/mirkov/data-table/blob/master/user/example1/README.org

That same directory contains examples-w-fft.lisp that shows the code 
usage.  I annotated the code.

The png files show sunspot data and two power spectra.

The example shows how I create one data-table with raw data, and then 
another one with slightly massaged data.  I do the fft on the latter one.

The example also shows that the current syntax is `functional' vs 
`declarative'. I explicitly  build the second data table, instead of 
declaring its dependency on the first table.  I think that the latter 
approach is preferable from a user's point of view.  But I will refrain 
from trying to design and implement this declarative approach.  I need more 
real-world experience before trying to do this.

I may add windowing capability to the fft code later on.

Mirko
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