anesthetic 3.0.0

[williamjameshandley]

We can use this issue to collect breaking changes that we may one day wish to make.

To kick things off:

1. moving of material from anesthetic.plot into anesthetic.plotting._matplotlib.
2. making levelled histograms the default for hist_plot_2d

[AdamOrmondroyd]

I've caught myself out a few times recently with ['nlive', 'logL', 'logL_birth'] not being part of the index so they're lost when slicing. They would make the index very wide though...

[appetrosyan]

Support for other rendering backends. Particularly https://github.com/bokeh/bokeh. This would require refactoring the code into a backend-agnostic part.