[Ifeffit] normalization methods

[Matt Newville]

Hi Folks,

Over on the github pages for larch, Mauro and Bruce raised an issue
about the "flattening" in Athena. See
https://github.com/xraypy/xraylarch/issues/44

I've added a "flattened output" from Larch's pre_edge() function, but
the question has been raised of whether this is "better" than the
simpler normalized spectra, especially for doing PCA and/or LCF for
XANES.

Currently, the "normalized" spectra is just "(mu -
pre_edge_line)/edge_step". Clearly, a line fitted to the pre-edge of
the spectra is not sufficient to remove all instrumental backgrounds.
In some sense, flattening attempts to do a better job, fitting the
post-edge spectra to a quadratic function.  As Mauro, Bruce, and
Carmelo have pointed out, it is less clear that it is actually better
for XANES analysis.  I think the main concerns are that a) it is so
spectra-specific, and b) it turns on at E0 with a step function.

Bruce suggested doing something more like MBACK or Ifeffit's bkg_cl().
 It would certainly be possible to do some sort of "flattening" so
that mu follows the expected energy dependence from tabularized mu(E).

Does anyone else have suggestions, opinions, etc?  Feel free to give
them here or at the github page....

--Matt