[Therion] Aligned cross sections for multiple passages

[Tarquin Wilton-Jones]

For anyone interested, I have documented my approach here:

https://therion.speleo.sk/wiki/multiplesections

Not that I have tested it extensively yet, so I may have to refine the
"including it in your survey" part. I do plan to add some screenshots
later, once I actually have used it for real.

If anyone has any suggestions for how to simplify or improve the
process, please let me know :)