allow creation of "Feff Path" from experimental data

[newville]

There should be an ExperimentalStandardPath function (possibly with a shorted name!) to convert an isolated chi(q) to a (virtual) FeffPath that can be used in feffit(). I think this needs arrays of k, amp, phase, Reff, and may take option path geometry so that the sigma2_eins and sigma2_debye functions may be used.