CIL-Demos test script fails on demos that import .py files

[lauramurgatroyd]

This is because the files do not get copied to the tmp directory.

E.g. 02_Dynamic_CT_tmp.ipynb tries to import from utilities_dynamic_ct.py and fails

[lauramurgatroyd]

WIP at https://github.com/TomographicImaging/scripts/tree/improve_nb_test_and_docs