Convert to BussProofs

## Description
The proof tree notation is done using `\cfrac`s in a maths environment, this is not ideal as:
- side tags are part of the width of the `\cfrac`
- no proper centering control

I recently did a CW with [bussproofs](https://mathweb.ucsd.edu/~sbuss/ResearchWeb/bussproofs/) and like this style.
In the long term, should probably move to using this.