What additional content do we need?

[MargaretDuff]

A chance to list what we would like to write new notebooks on:

• Callbacks
• Step size methods
• Preconditioners
• • https://github.dev/epapoutsellis/CIL_work/blob/master/Explicit%20PDHG%20precond%20vs%20Explicit%20SPDHG%20precond.ipynb
• Phase contrast
• Examples of setting and getting centre of rotation, including example of a tilted rotation axis