Remove tomophantom requirement

[lauramurgatroyd]

Add shepp-logan to CIL-Data or use another package to generate it

[hrobarts]

https://scikit-image.org/docs/0.25.x/api/skimage.data.html#skimage.data.shepp_logan_phantom

[lauramurgatroyd]

or could use from astra, which allows geometry definition: https://astra-toolbox.com/apiref/data2d.html#astra.data2d.shepp_logan

[lauramurgatroyd]

agreed not to, see TomographicImaging/CIL#2218 (comment)