Add XML SkyModel serialization

[joleroi]

This PR adds support for XML serialisation of sky models.

[cdeil]

@joleroi - Thanks!

I think my preference would be to do this:

from gammapy.cube.models import SkyModels
models = SkyModels.read('models.xml')

and basically have this:

class SkyModels:
    def __init__(self, model_list):
        self._model_list = model_list

It's close to what you put here, but it

• wraps a list instead of subclassing list - that means we can put any API we like, and start by exposing a minimal API
• exposes the I/O in a class / place that's nice to find for users - read would just call into gammapy.utils, but I think for users it's a bit nicer to have that factory method accessible with the models.

[joleroi]

I'm -1 on SkyModels to close to SkyModel will confuse everyone, no? It's analog to Observation and ObservationList, isn't it? So I'd like to stick with SkyModelList. I'm fine with having a list as data member not subclassing it.

[cdeil]

We've had these discussions before. "List" shouldn't be in the name if it's not a list, but a wrapper that doesn't really expose the list API (like slicing, ...).
I could dig up issues that result from subclassing, e.g. HDUList is a list subclass, but then some list methods don't work properly, e.g. .copy() gives crazy results.
There's a lot of things you have to think about and take care of when you make a list, and when callers start using the list API you're bound to it.
In that sense, exposing a minimal API is the safe way to start, and then it's easier to change to something else later.

My preference would be SkyModels and Observations for the containers, but I'm also OK with SkyModelCollection or SkyModelLibrary or something like that if you prefer.

[joleroi]

No need to argue about not subclassing list, I already agreed 👍

We've had these discussions before. "List" shouldn't be in the name if it's not a list, but a wrapper that doesn't really expose the list API (like slicing, ...).

Also agreed, I will thing about a name, and fall back to SkyModels if I cant't come up with anything

[joleroi]

I didn't push anything to gammapy-extra yet. This old version of the diffuse model is ~30 MB. OK to add? Or do you have a better idea for a test case?

[cdeil]

There is e.g. gammapy-extra/test_datasets/unbundled/fermi/gll_iem_v02_cutout.fits.
Let's not add a 30 MB file.
IMO even better would be to not use data files if not really needed, and instead create the file in a temp folder in the text with a few lines.
With gammapy.maps it's now easy to make any kind of map; for example:
https://github.com/gammapy/gammapy/blob/master/gammapy/cube/tests/test_models.py#L14

[joleroi]

👍

[joleroi]

This will be part of this PR