Add notebook interactions to spec1d cubeviz example

[eteq]

This adjusts the cubeviz notebook and some of the corresponding code to demo using the notebook idiom to combine an "interactive" and "notebook" usage.

1. It fixes up the units from the original data file to be in angstroms. To do this I had to do a slightly hacky thing in the cubeviz initializer, but I think it's probably fine as a stand-in. Moving forward we'll probably want to instead allow passing in data objects instead of names, but this seemed the fastest route forward. Note @brechmos-stsci - if you want to keep using the "fixed" file, you could just delete the process_data part and I presume it'll be the same
2. It adds some analysis using specutils at the bottom of the notebook.
3. it renames getRegions to get_regions since that's now showcased in the notebook

The last cell is meant to be used with the interactive selection in the profile viewer. I.e., it's just empty unless the user selects something in the profile view.

[brechmos-stsci]

Basically worked, but the last cell still has getRegion rather than get_region. Could you change that and then I'll commit this.