Add entry points for viewer registry

[jfoster17]

Populate viewer registry for all viewers

Description

This uses entry points to register all the viewers for glue-jupyter into viewer_registry. This addresses #388 and makes all viewers accessible through the new_data_viewer() command in glue-jupyter. I think this should be compatible with #402, because the viewer registry won't double-populate the same viewers. I have added IpyvolumeScatterView as scatter3d.

Adding an entry point for glue-wwt will be a separate PR in that repository.

Not 100% about the names here. 'image' is a convenient name for calling in code, but is less descriptive than something like '2d image'. This is particularly relevant for 'scatter' (unqualified) versus 'scatter3d'.