Create tutorial about imglib2-roi and how to work with labelings

Documentation on the most recent features of `imglib2-roi` is still sparse. Some of it is in the [`roi-demo`](https://github.com/imglib/imglib2-roi/tree/roi-demo) branch, some other info is scattered across github issues or on gitter (see e.g. [here](https://gitter.im/imglib/imglib2?at=5c542b97975714406b7838d2)).

Let's add a tutorial here (notebook or Maven project?) summarizing the functionality.

Start by reviewing the roi-demo branch and the linked GitHub and Gitter discussions to gather the recent imglib2-roi features and labeling guidance. Decide whether the tutorial should be a notebook or Maven project; done means a usable tutorial here that summarizes the relevant functionality.

Written by the indexing model from the issue text.