Integrate napari layer group into atlas viewer plugin

Currently, we just add each part of the atlas we are interested in as a separate layer to napari. In the future, the napari representation of a BG Atlas should be a layer group containing all the layers of one atlas together.

Start by tracing how the plugin currently adds each atlas part as a separate napari layer and how a BG Atlas is represented. Confirm napari's layer-group API and identify the plugin entry point involved; the work is done when one atlas appears as a layer group containing all of its layers.

Written by the indexing model from the issue text.