Old tutorials - update multiple imaging tutorial

[drannawatts]

We have two tutorials (one on multiple imaging, and a deprecated tutorial on postprocessing) in the 'old' directory of the documents. We should either retire these or incorporate them properly back into the new documentation.

[drannawatts]

Modifying this now to focus on multiple imaging, since the postprocessing one is dealt with by pull request #488

[thjsal]

The tests done in multiple imaging tutorial were by the way referenced in the footnotes of Riley+2021.

But the link to this is anyway not working, so perhaps it is fine to not keep it it archived for an infinitely long time.