Image from catalog

Incomplete, work in progress - I made a pull request to allow discussion about the structure & implementation.

Note that some functions also use stuff used in npred_cube2 pull request (which is why I rebased against it in this PR).

To do (Ellis):
- [x] Check output type & format looks sensible
- [x] Ensure output values are sensible (Flux normalization)
- [x] Write unit tests
- [x] Add doc strings
- [x] Add short tutorial
The flux values are about 30 orders of magnitude larger than I expected for the extended image, and I don't know why...
However, I don't need this function anymore so I don't think there's any point in implementing it now - shall I put a not implemented error or just remove the code?