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
It's good if example code is short, so I'd suggest to delete these variables write here and simply write:
```
reference = make_empty_image(nxpix=100, nypix=100, binsz=1)
```