Add an ExtendedComponent to glue

[jfoster17]

Is your feature request related to a problem? Please describe it:
Some datasets such as geographic entities, features on the genome, and regions-of-interest in an image (identified algorithmically) inherently contain information about their extent in a way that glue should be able to visualize and reason about. For anything except a tiny dataset it is not feasible or desirable to translate all these extents into glue subsets.

Describe the solution you'd like:
Add an ExtendedComponent type to glue that would allow for a component to specify a shape (or set of shapes) for each element of a dataset. ExtendedComponents would describe an extant on one or more other Components in the dataset (parent_components— which may be CoordinateComponents or regular Components), and should contain references to those parent_components. Data Loaders would be responsible for parsing input data files into the appropriate components and individual Viewers would be updated to support plotting ExtendedComponents. ExtendedComponents should perhaps be hidden from the user and only used by Viewers when plotting the parent_components and transforming ROIs to SubsetStates.

Describe alternatives you've considered:
The following packages provide two partial and alternative approaches to this problem:

• Geographic regions (e.g. https://github.com/jfoster17/glue-map) -- uses a custom Data class which includes geometry and centroid attributes to represent the extent of geographic regions.
• Regions-of-interest in astronomy images (e.g. https://github.com/glue-viz/glue-regions) -- casts regions as subsets

Additional context:

The biggest decisions to be made are:

• How to handle ROI selections that partially intersect with an ExtendedComponent
• How to deal with the conversion that a user will probably want to make from a subset over data points (i.e. states) to a subset over the extent of that data points (i.e. the geographic region contained within a set of states)

See the attached overly long PDF for my full thinking through of what this might look like.

ExtendedComponent.pdf

Related Issues

• Feature Request: ds9 regions #320
• Add support for recognizing geographical regions #2086
• Support reading from astropy regions objects glue-astronomy#55
• New component/attribute type for geospatial regions #1686

[jfoster17]

I'm going to take a first pass at refactoring glue-map to use this kind of approach and see how that goes...