(disabled) ability to "freeze" subsets by converting to masks

[kecnry]

Description

This pull request is to implement support for "freezing" subsets to masks.

TODO:

• support in app.get_subsets() and the subset tools plugin (masks should show as an entry, even if the only information provided is that there is a masked subset in the list of sub-subsets).

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)?

[pllim]

I thought we discussed this and decided that this is too resource intensive? You have to read all the image pixels to memory even if your mask is very small (though I don't grok all the stuff you copied from glue there)?

Also see glue-viz/glue#2415 (review)

[kecnry]

this is not for the same purpose and so would not be done automatically ever, but only when the user intentionally wants to create a mask. Right now, the only obvious use-case for this is in lcviz (spacetelescope/lcviz#45) and therefore support in jdaviz is disabled by default unless configs enable it.

[kecnry]

Superseded by #2462