Add an example for adding a segmentation/mask to a dataset

[hotzenklotz]

Detailed Description

• Affected library: e.g. webknossos

Add a simple example for adding a segmentation/mask/binary mask to a dataset with one color layer. Can be a very short snippet but some users did not understand the difference between color / segmentation layers straight away.

Use Cases & Context

A user has an image/dataset and a corresponding mask either as Tiffs or Numpy arrays. How do you convert that to WK datasets or add a mask to an existing dataset.