[Ifeffit] weights data processing

[Matt Newville]

Stefano,

Just to follow up on Scott's explanation on multiple k-weights (I 
have nothing to add to his discussion on k-weights in spline()):

> 2) In artemis, is it a bug or a wanted feature to have the 
> possibility to activate more than one button for k-weight for the fit 
> and FT? I suppose it is a wanted feature, but then I wonder what is 
> the meaning of using more than one weight for the fit and FT. I am 
> sure some of you can unveil this enigma.

When multiple k-weights are selected, they are _ALL_ used.  That
is, for each k-weight an FT with that k-weight is done, and the
resulting chi(R) spectra are appended together.  The refinement is
then done on the appended spectra, using all k-weights.  This is
essentially the same as refining multiple data sets. In fact,
until very recently, the only way to use multiple k-weights was to
use multiple data set fits where the only thing that changed
between data sets was the k-weight.  The new way (and checking
multiple boxes in artemis) makes the move from 'possible, in
principle' to 'easy, in practice'.

As for why you'd want to do this, Scott's explanation is right on.  
I might go so far as to recommend using k-weight = 1 _and_ 3 as
the normal way to analyze XAFS data. 

For what it's worth, the wavelet transforms being used in some
XAFS programs (see, for example,
http://www.esrf.fr/exp_facilities/BM20/Software/Wavelets/) is a
more sophisticated way of doing multiple k-weightings, and may
give you a good visualization about why you'd want to do this. You
might even view multiple k-weight fitting as a poor approximation
of wavelet transforms.

Hope that helps, 

--Matt