Describe Overhauser splines

[mgeier]

They are already mentioned several times in the text because they are often confused with Catmull-Rom splines.

The relevant paper is already part of the references:

https://github.com/AudioSceneDescriptionFormat/splines/blob/f993a797ee7c530608258ed0a6b98999829e499a/doc/references.bib#L80-L89