Including Roughness-Parameters as material properties

[TimoHinsemann]

As discussed in the group meeting, we would like to add the parameters "Surface height root-mean-square (RMS-Roughness)" and "Surface (auto-)correlation length" as material properties. The precise wording could be also taken from:

Reasoning:

• Following formula being already used for radar simulation, with "sigma" being the RMS-Roughness and "m" being dependent on autocorrelation length.

• The two parameters being generally used for diffraction-based modelling with gaussian-surface assumption: https://stars.library.ucf.edu/etd/1086/?utm_source=stars.library.ucf.edu%2Fetd%2F1086&utm_medium=PDF&utm_campaign=PDFCoverPages

Roughness-Measurement:

• https://www.mitutoyo.com/webfoo/wp-content/uploads/1984_Surf_Roughness_PG.pdf
• https://www.qualitymag.com/ext/resources/files/white_papers/Olympus/Introduction-to-Surface-Roughness-Guide.pdf?1611866522
• https://get-it-made.co.uk/resources/surface-roughness-explained

[KimuraDIVP]

surface_roughness should be [optional], not [required]. We don't use roughness in our simulaor. 

[KimuraDIVP]

From the standardizaiton point of view, Ra will be better than RMS(Rq).

[KimuraDIVP]

Maximum height "Rz" will be required.