GUI real space: show roughness

Roughness is an important property of material interfaces, but is not yet represented in our real-space sample view.

Research from a quick&dirty literature search:

A recent paper on how to numerically generate rough surfaces with given spectrum: Perez-Rafols & Almquist 2019, https://doi.org/10.1016/j.triboint.2018.11.020

Rough surfaces generated with closed-source software Comsol: https://www.comsol.com/blogs/how-to-generate-random-surfaces-in-comsol-multiphysics/

Simple Matlab code, without bothering about some of the difficulties discussed by Perez-Rafols & Almquist: https://www.mathworks.com/matlabcentral/fileexchange/60817-surface-generator-artificial-randomly-rough-surfaces