Restore scan intensity

Specular scans measure reflected or transmitted intensity relative to the incident intensity. Otherwise said, the incident intensity is always 1.

Nonetheless, an intensity variable is needed as a fudge factor for use in fitting experimental data. It needs to be restored to core and connected with the GUI.

added GUI release-critical labels

changed title from GUI: Specular simulation has no intensity variable to Restore scan intensity

changed the description

assigned to @j.wuttke

mentioned in merge request !1350 (merged)

mentioned in commit 824df3bc

closed with merge request !1350 (merged)