add optional sidecar variable to imviz class

[pcuste1]

Description

This PR adds an optional sidecar variable to the imviz class that will be used to hold the current sidecar instance that imviz is loaded into, if it is loaded into a sidecar instance.

The PR is required for spacetelescope/mast-aladin#73

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?
• If new remote data is added that uses MAST, is the URI added to the cache-download.yml workflow?
• 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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 87.63%. Comparing base (fbcabce) to head (aa96ca8).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3850   +/-   ##
=======================================
  Coverage   87.63%   87.63%           
=======================================
  Files         188      188           
  Lines       25703    25704    +1     
=======================================
+ Hits        22524    22525    +1     
  Misses       3179     3179           

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