save_as_png AttributeError

Hi, I am using higlass v0.4.0 and I'm trying to save a png file in a jupyter notebook as [described in the tutorial](https://docs-python.higlass.io/getting_started.html#saving-the-view). However, I get the following error message:

```AttributeError: 'HiGlassDisplay' object has no attribute 'save_as_png'```

Any troubleshooting suggestions?

Read the linked saving-the-view tutorial and inspect the HiGlassDisplay API exposed by v0.4.0. Reproduce the AttributeError in a minimal Jupyter notebook, then compare the documented method with the installed package. Done means the tutorial's save-as-PNG path works or the documentation clearly matches the supported API.

索引モデルが issue の本文から書いたものです。