Update nomenclature for SESANS plots

[gnsmith]

I am the chair of the canSAS SESANS Reproducibility Working Group.

After much discussion in our group, we recently agreed recommendations for nomenclature for SESANS measurements. This is an update of previous nomenclature, but we feel that these should stick for now. Hopefully! There was at least agreement from the three facilities that run SESANS instruments...

The paragraph about it is on our wiki page.
https://wiki.cansas.org/index.php?title=SESANS_Reproducibility_Working_Group

There are two main changes that would need to be made in SasView. The variable for spin-echo length needs to updated from z to δ. The unit for both length scale and wavelength needs to be changed from A to Å.

I just checked and the Å (ångström) rather than A (amp) thing is also the case in the normal SAS window.

[caitwolf]

Changing units from A to Å would be a larger change that affects more than SESANS data so I've created another issue #2753.