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

[imagejan]

Documentation on the most recent features of imglib2-roi is still sparse. Some of it is in the roi-demo branch, some other info is scattered across github issues or on gitter (see e.g. here).

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

[ctrueden]

There are a couple of imglib2-roi-based exercises written by @maarzt here and here.

I am not sure where any ImgLabeling etc. examples might reside.

[ctrueden]

Check out new functions from imglib/imglib2-roi#48 to see if they belong in this tutorial.