[Transmission Samples] Add absorbance step

[maurov]

@newville I take the occasion to say that xrayweb is very helpful and it is a nice alternative to Hepheastus or XAFSmass. In the "Transmission Samples" tab, would it be possible to add the calculation of the absorbance step for the element closest to the given energy and show it in the "Transmission Sample Results"?

[newville]

@maurov would it be helpful to have a selection for element and edge, and then show the absorbance above and below?

[maurov]

Personally I like the way is done in XAFSmass, but what you propose is fine too.

[newville]

@maurov @easyXAFS I think no one would be opposed to showing all the variations of how to present this. Are you willing to make a PR or even write down the calculation you want to see?