Generalize specviz/2d load_data

[kecnry]

Description

This pull request generalizes load_data in specviz to parse 1d and 2d spectra as well as specreduce trace objects. This does not yet allow visualizing or analyzing 2d spectral data in specviz, but lays the groundwork for adding those viewers/plugins. Because of that, this is temporarily behind a developer flag:

specviz._dev_deconfig = True

Note that this is not the intended eventual design of the general parsers, but rather a temporary (slightly hacky) solution to get the existing parsers in specviz2d to work in specviz so that we can continue with the rest of the implementation of context-aware viewers and plugins which then will hopefully help inform the refactored parsers in the future.

TODO:

• hide 2d cases behind developer flag
• deprecate specviz2d.load_trace
• deprecate specviz2d.specviz

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)?

[kecnry]

as far as I can tell, this parser hasn't been used since we began using the spectral extraction plugin in specviz2d