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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 docstring convention for string options is
```
catalog : {'1FHL', '2FGL', 'simulation'}
```
and not to put
```
catalog : str
```
 and the list the options in the description.

There's a few places where this needs to be changed ... please read over the diff on Github and fix this everywhere.