Add tutorial/docs for visualization

[astrofrog]

Once #29 is merged, I will work on this.

[astrofrog]

(this is still work in progress - I'll post a link to a built version on RTD once it's ready for review)

[astrofrog]

@ChrisBeaumont - this is ready for review, but readthedocs is acting up (I know it builds with the correct dependencies because it worked once, but it's refusing to build on demand for my fork) so you'll need to build locally to see the new documentation page. Let me know what you think about the plotting API!

[ChrisBeaumont]

Looks great! This puts the old IDL code to shame :)

One minor point: in stead of d.plotter().plot_tree(ax, ...), what do you think about d.plotter(ax).plot_tree()?

Also, is this compatible with plotting with APLpy?

[astrofrog]

Hmm, that's not a bad idea. In that case, the plotter becomes single purpose (i.e. tree or contour). Maybe I should then make two separate Plotter classes since only the tree one needs the cached positions? Then one would avoid recomputing the cached positions when calling plotter(ax) to make the contour plots. What do you think? I'm flexible on this.

[astrofrog]

The problem with the latter option is then once has e.g. d.contour_plotter.plot_contour which is a little redundant, so maybe not a good idea...

[ChrisBeaumont]

You're right, that doesn't feel quite right. The way it is is fine

[astrofrog]

Regarding APLpy - I'm planning to try and make it possible to use them together, and will add a section here once I get an example working :) (will open an issue for that)

Is this ok to merge?

[ChrisBeaumont]

👍