Reduce duplication across user guide for "parameterisation" sections

[gbfredrik]

Hi, We were looking at making some contributions to the user guide but found it a little difficult with the many (sub-)sections being split a little differently. A few of the files here, Docs/UserGuide/parameterisation, were not included into the main .tex file. This meant that edits had to be made in two places to prevent the sources from being out of sync.

In this PR, I have taken the most recent changes from the main "userguide.tex" file and pasted them into the sub-files, to ensure nothing is lost. Then there will be one golden source for each section.

What do you think, is this okay to do? :)

Thanks a lot! Fredrik