Ensure there are references where needed

[dbosk]

Currently some statements might benefit from the backing of a good reference. We should read through and look for weaknesses in the argumentation.
Reveiwers comments:
ChapterComments2.pdf, p 3

[dbosk]

Note down where we lack references and preferably find good references to back up our statements.

[dbosk]

There is still a TODO for this in intro.tex.