Cubeviz spectral extraction to ignore NaNs by masking them out

[pllim]

Description

This pull request is to address problem of extracted spectrum being NaN when cube has NaN.

I cannot find option to do this upstream in astropy.nddata. Example code upstream: https://github.com/astropy/astropy/blob/47a8dac8f1dab6ce592be6048f70140adead4e1d/astropy/nddata/mixins/ndarithmetic.py#L633-L634

This is also a problem in v3.8.x but direct backport is probably tough with the recent refactoring. If you really want to backport, please change the milestone and set backport label.

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]

Let's get this in for now and we can always remove it if we get a "proper" fix upstream. Thanks for the quick fix!

[gibsongreen]

This looks good to me!

[pllim]

Thanks for the reviews!