02_sandstone notebook show geometry plot is confusing

[MargaretDuff]

The plot shows the detector half way through the object and this is confusing to people attending training:

[MargaretDuff]

We had about 6 questions on this at the ICMS hackathon in Edinburgh

[lauramurgatroyd]

We can do similar to what we did in the preprocessing 03 notebook in this PR

https://github.com/TomographicImaging/CIL-Demos/pull/238/changes