Implement edge alignment

[matthewcarbone]

Zhu's original comments:

Edge alignment. This include the following steps:
Perform 1~4 for reference spectra (labeled as “ref”), and find the relative edge position for them. These are usually measured on the same material.
Edge shift. Align the transmission and fluorescence spectra( labeled as “trans” or “flu”) by the edge shift found in their corresponding reference spectrum.

We should itemize this a bit more finely. In other words, perhaps allow the user to provide the spectra they want to shift.