Create documentation about layer artists

[astrofrog]

@ChrisBeaumont - while working on this, I ran across a couple of things in the API that would be nice to update:

• DataViewer._container: this name is pretty obscure, and it should maybe instead be DataViewer._layer_artists or DataViewer._layer_artist_container.
• Maybe the layer artist container should have a consistent name between clients and widgets (at the moment it's .artists vs _container for the same object, and artist_container in some kwargs). What about .layer_artists, and make it public for both clients and widgets?
• I have a mental block when I run across instances of the layer variable which really is the layer data. Would you have any objections to me renaming layer to layer_data? So then we'd never have just 'layer' - either layer artist or layer data.

What do you think?

Side note: at the moment, we don't really talk about the possibility of using clients in these docs - and that's because in principle one doesn't have to use a separate client class. Before writing docs on how to better separate client functionality and GUI, I think it would be good to converge on a solution in #775 first, so adding details of clients to the page here is low priority for now in my view.

@PennyQ - just for info, I've started working on this, but it's by no means complete. I'll work on it more over the next couple of days (in my spare time).

[ChrisBeaumont]

Doc changes LGTM.

DataViewer._container: this name is pretty obscure, and it should maybe instead be DataViewer._layer_artists or DataViewer._layer_artist_container.

DataViewer._layer_artist_container sounds good

I have a mental block when I run across instances of the layer variable which really is the layer data. Would you have any objections to me renaming layer to layer_data?

I can't remember, but doesn't layer sometimes refer to a Subset as well? In that case layer_data sounds misleading to my ears (as I recall I think I started using layer as an alias for subset or data, but I agree this isn't very obvious)

[astrofrog]

I can't remember, but doesn't layer sometimes refer to a Subset as well? In that case layer_data sounds misleading to my ears (as I recall I think I started using layer as an alias for subset or data, but I agree this isn't very obvious)

Indeed it can refer to a subset. I'll try and think of whether there is a clearer name that would better encapsulate this, but otherwise I'll leave it as-is.

[astrofrog]

I'll go ahead and merge this, and will continue working on the docs for the Qt data viewer in future (but what's added in this PR is already fine).