[upd_gui_projects] Upd GUI projects 20.0, 20.2, 21.0, rm distrubutions from depthprobe and offspec ()

Merging branch 'upd_gui_projects'  into 'main'.

See merge request !2002

[j.715a] Before starting scattering simulation check that refractive index has reasonable magnitude (#715) (Closes #715)

Merging branch 'j.715a'  into 'main'.

See merge request !2001

Merging branch 'i753'  into 'main'.

See merge request !1997