k-grid in feffNNNN.dat

[bruceravel]

It's hard to say this better than Matt put it in an email to me:

...we should test ... the "write feffNNNN.dat / read feffNNNN.dat"  steps, which 
are kind of weird, really.  chi(k) is broken apart on a strange k-grid, a bunch of 
arrays are down-sampled to single precision, and written out with even less 
precision.  Then we read in those results, interpolate the components of phase/
amplitude as arrays of k and then recompute chi(k) to compare to data.   It might
be fine, but I think the last time this was looked at seriously was 1991.

So ... make the k-grid on which the arrays are saved configurable.

[bruceravel]

It turns out that we cannot yet address this point. genfmt and onepath get the k grid from the phase.pad file. See https://github.com/xraypy/feff85exafs/blob/master/src/GENFMT/genfmt_prep.f#L72

It seems that changing the k-grid is something that has to be done at the level of a POT/XSPH wrapper.