display units: cubeviz/slice plugin

[kecnry]

Description

This pull request enables the unit conversion plugin in cubeviz and adds support for the slice plugin/indicator. The slice slider is always in order of increasing slice, so will not necessarily correspond to the positive x-direction on the viewer if switched from wavelength > frequency (or vice versa, depending on the original native units).

Screen.Recording.2023-04-05.at.2.13.42.PM.mov

Note that we might want to generalize some of this logic more into the mark's _update_data when adding support for spectral lines as well.

Change log entry

• Is a change log needed? If yes, is it added to CHANGES.rst? If you want to avoid merge conflicts,
  list the proposed change log here for review and add to CHANGES.rst before merge. If no, maintainer
  should add a no-changelog-entry-needed label.

Checklist for package maintainer(s)

This checklist is meant to remind the package maintainer(s) who will review this pull request of some common things to look for. This list is not exhaustive.

• Are two approvals required? Branch protection rule does not check for the second approval. If a second approval is not necessary, please apply the trivial label.
• Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• Do the proposed changes follow the STScI Style Guides?
• Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• Did the CI pass? If not, are the failures related?
• Is a milestone set? Set this to bugfix milestone if this is a bug fix and needs to be released ASAP; otherwise, set this to the next major release milestone.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[duytnguyendtn]

Code looks like code, and the slider on the spectrum viewer works as expected. Also, great to see the slice plugin was also caught. Changing to Hz also changes the "direction" the plugin slider moves, which I would expect

[pllim]

Does this need glue-core v1.9.0 because we cannot right now, see #2136

[kecnry]

Does this need glue-core v1.9.0?

Yes, that will likely block the eventual merging of #2127 into main, but should still be able to be tested locally with glue 1.9 or glue-viz/glue#2379.

[javerbukh]

Looks good!