add support for c1d / x1d multi science extension

[Jdaviz-Triage-Bot]

Reporter: Brian Cherinka

Jdaviz needs support for c1d and x1d data products that contain multiple science extensions.  For x1d products, these have multiple EXTRACT1D extensions.  For c1d products, these have multiple COMBINE1D extensions.   This work is follow-up work from https://jira.stsci.edu/browse/JDAT-1471 , https://jira.stsci.edu/browse/JDAT-1470 , and #621.

As these files are single files that have multiple science extensions, a minimal use case is to open the file up in Mosviz, which displays a single spectrum profile viewer plus a table viewer.   I don't know if the Mosviz Refactor might contain a more specific loading configuration based on instrument and exposure type.  

🐱 

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DISCLAIMER: This issue was autocreated by the Jdaviz Issue Creation Bot on behalf of the reporter. If any information is incorrect, please contact Duy Nguyen

[pllim]

This is addressed by astropy/specutils#856 and #843.

[havok2063]

Reopening as this is not addressed by those tickets.

[havok2063]

Actually #843 is the correct one.