add support for scientific notation in unit string

[gibsongreen]

Description

This pull request is to address when scientific notation (a scale factor), is included in the native unit of the data file parsed (eg. in a MANGA cube). It is addressed by updating the values of the data to reflect the scale factor, and in the Unit Conversion plugin/app-wide, 'removing' the scale factor from the actual unit. When viewing the BUNIT metadata, the scale factor is still included in the unit.

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)

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• 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

All modified and coverable lines are covered by tests ✅

Project coverage is 88.50%. Comparing base (32bb09e) to head (783db28).
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3200   +/-   ##
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  Coverage   88.50%   88.50%           
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  Files         124      124           
  Lines       18577    18579    +2     
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+ Hits        16441    16443    +2     
  Misses       2136     2136           

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[kecnry]

LGTM - love the simplicity!

[kecnry]

my only concern here is if there was a case that fell within the tolerance because of an actual scale in the unit, but I don't think that's a realistic real-case scenario and this is probably a better idea than checking ==1 anyways.

[kecnry]

Pausing this for a bit since #3192 would technically avoid the need for this by adding the native units into the list of choices for flux display units. I still think we might want this, but let's test this vs #3192 vs the combination of the two and decide the pros/cons before merging.