BUG: Fix parsing of MANGA cube (try 2)

[pllim]

Description

This pull request is to fix loading MANGA cube in Cubeviz. Since Brian C said it works on Jdaviz 3.10 series, probably no need to backport.

This supersedes #3115 and takes advantage of the loaders that already exist in specutils when we can, like Brian Cherinka suggested. Also convert uncert type to stddev when we can, like Brett Morris suggested.

I tried to get rid of _parse_hdulist altogether but too many tests failed, so I kept it as a fallback.

Also fixed a Marker display bug where wavelength shows as 0.000 m, which is very uninformative.

Change log entry

• [x] 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.

• [x] 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.
• [x] Do the proposed changes actually accomplish desired goals? Also manually run the affected example notebooks, if necessary.
• [x] Do the proposed changes follow the STScI Style Guides?
• [x] Are tests added/updated as required? If so, do they follow the STScI Style Guides?
• [x] Are docs added/updated as required? If so, do they follow the STScI Style Guides?
• [x] Did the CI pass? If not, are the failures related?
• [x] 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.
• [x] After merge, any internal documentations need updating (e.g., JIRA, Innerspace)? 🐱

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 88.89%. Comparing base (5a020cc) to head (b80fb8a).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #3119 +/- ## ========================================== + Coverage 88.88% 88.89% +0.01% ========================================== Files 112 112 Lines 17391 17396 +5 ========================================== + Hits 15458 15465 +7 + Misses 1933 1931 -2 ```

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[pllim]

@havok2063 , would this work for your use case? Can you please try it out? Thanks!

[pllim]

Rebased to get rid of merge conflicts.

[pllim]

Thanks for the reviews!