imviz-round-1.5 (astrowidgets + Glupyter)

[pllim]

This is like #428 but swaps out Ginga with Glupyter. Sketchy dummy classes are gone. I was unable to test the CLI locally due to voila-dashboards/voila#843 (long story... see #448 )

Special software requirements:

• Dev version of glue-jupyter
• Latest bqplot-image-gl might work though I used dev version too
• astrowidgets from EXP: glupyter implementation on abstract class astropy/astrowidgets#131 that builds on Initial implementation of abstract class astropy/astrowidgets#126 (until these are merged, the CI here won't pass)

Known issues:

• Mouse interaction for cursor info panel breaks when you use scroll/pan GUI from the toolbar.
• To regain mouse interaction, you need to go into Data layer to uncheck/check the data name, which resets everything.
• If you use the astrowidgets API to load data, it clears the previous data collection first, otherwise the API doesn't work properly; see "Known issues" at EXP: glupyter implementation on abstract class astropy/astrowidgets#131 -- Should not be an issue unless we want to load multiple datasets into the same viewer?

TODO

• Get feedback: Does this make sense? Can @astrofrog build on this? Does he want to?
  • Tom R does not think this is the right approach, see Widget should not inherit from VBox astropy/astrowidgets#132
  • Erik T said we can emulate specviz/helpers.py but is it the the mixin or the API extension in final subclass?
• How can scroll/pan from toolbar not break cursor info panel mouse iteraction? Maybe if we apply feat: compose MouseInteraction with other interacts glue-viz/bqplot-image-gl#45 somehow
• Try out other stuff on the toolbar (region selection)
• Address TODO over at EXP: glupyter implementation on abstract class astropy/astrowidgets#131

[pllim]

If we decide to revisit astrowidgets in the future, will build off #429 instead.