Drag and draw annulus with hardcoded width

[Jdaviz-Triage-Bot]

Reporter: pllim

As proposed by @kecnry , here is the desired workflow:

1. User select new shape tool from the existing shapes drop-down.
2. User click and drag to draw. At this point, it is basically the circle draw tool.
3. When finalized, this tool magically creates an annulus shape with hardcoded width (proposed: inner_radius = outer_radius * 0.5).
4. To modify the annulus shape, user has to edit it manually in the Subset Tools plugin.

New features to be implemented:

a. A new shape tool that subclass the circular shape but then turns it into annulus when finalized.
b. Subset Tools plugin needs special logic so that user can edit only the following for annulus (which is a very special case of compound subset): One center, inner radius, outer radius. @javerbukh proposed maybe we can subclass subset state or attach metadata to the state.

Under the hood:

Annulus subset state
|___ ROI state --> CircularROI
|___ Invert state --> ROI state --> Circular ROI

where

subset_state.state1.roi.xc == subset_state.state2.state1.roi.xc

and

subset_state.state1.roi.yc == subset_state.state2.state1.roi.yc

and

subset_state.state1.roi.radius > subset_state.state2.state1.roi.radius

Also see https://github.com/glue-viz/glue-astronomy/blob/336e2cca4a93025e34e4a0d4313b4031f2f794dc/glue_astronomy/translators/regions.py#L52

This needs new glue-astronomy release upstream (>0.7).

🐱

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