subset tools import_region to use batch_load

[kecnry]

Description

This pull request delays loading subset layers into viewers during subset tools import_region until all regions are loaded into the data collection, improving performance considerably.

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. Bugfix milestone also needs an accompanying backport label.
• After merge, any internal documentations need updating (e.g., JIRA, Innerspace)?

[codecov]

Codecov Report

Attention: Patch coverage is 92.98246% with 4 lines in your changes missing coverage. Please review.

Project coverage is 88.82%. Comparing base (3dc6573) to head (ea9deba).
Report is 8 commits behind head on main.

Files with missing lines	Patch %	Lines
...nfigs/default/plugins/subset_tools/subset_tools.py	92.00%	4 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3321   +/-   ##
=======================================
  Coverage   88.81%   88.82%           
=======================================
  Files         125      125           
  Lines       19041    19044    +3     
=======================================
+ Hits        16912    16915    +3     
  Misses       2129     2129           

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[gibsongreen]

Code changes look good to me and the speed up is very noticeable, this'll be very nice to use with this use case!

[javerbukh]

Looks good and I agree that there is a noticeable speed up, thanks!