Documentation updates

[tovrstra]

There are several parts of the documentation that can be improved and updated, and recent publications and theses can be used as a starting point. I'll keep a list here that I can update whenever something comes to mind. None of these are urgent, just nice to have:

• An improved derivation of the five deviatoric pressure tensor components for shear viscosity calculations. (Also related to Generalize the ptens script for the assymetric tensor case #31).
• A nicer introduction to the basic concepts in continuous and/or discrete time and frequency domains, with a cleaner connection to the Wiener-Khinchin theorem.
• Improved notation for the Lorentz model and the interpretation of time scales.
• A cleaner introduction to the sampling error on a time-correlated average.