Refactor specviz parser

[Jdaviz-Triage-Bot]

Reporter: pllim

In general, the specviz parser is messy. Handling of spectrum list results in a lot of special logic route. I think things like these can be cleaned up:

• get rid of casting np.array(list of Quantity) because it looks unnecessary and cause the results to lose unit that later have to be manually re-attached, which also is prone to subtle bug if there is a case where inputs have mixed units and also defeats the purpose of passing in Quantity in the first place
• get rid of double for loop in Python; can probably be vectorized
• see if handling of spectrum list vs single spectrum can be made more elegant in general to ease future maintenance

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