spectral extraction: updates for Spectrum1D support in specreduce

[kecnry]

Description

This pull request implements the changes in jdaviz needed to make use of the improvements in astropy/specreduce#144 to add support for specutils.Spectrum1D support. This will also enable us to support keeping the spectral-axis/WCS throughout the spectral extraction process (although that is temporarily disabled here until the proper plotting support is implemented upstream).

Once specreduce 144 is merged and released, this PR should pin that new release and should then be tested (although it does currently seem to work as expected with the current state of the PR branch).

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)?