[Ifeffit] rebinning data

[Matt Newville]

A little while ago, Bruce said:

>     read_data(foo.dat, type=xmu, group=a)
>     spline(a.energy, a.xmu)
>
>     bin(a.energy, a.xmu, group=b, <<other binning parameters>>)
>     spline(b.energy, b.xmu)
>
>     newplot(a.k, a.chi, key=unbinned)
>        plot(b.k, b.chi, key=binned)

OK -- I agree with this.  There should be a binning command that
puts the data to a pre-defined x-array with some optional
parameters for the interpolation scheme (say, whether to use
boxcar averages or Lorenztian convolution) and how to deal with
repeated data, etc.

--Matt

PS: I hope to catch up with other ifeffit emails on multiple
k-weights and So2 v. Ei soon!!