add specviz property to cubeviz/mosviz helpers

[eteq]

This implements the feature I suggested in #329. This PR should probably also include some doc updates showing its use a la what @PatrickOgle originally was thinking about in #257 (comment). The idea is that with this you can do something like:

cubeviz = CubeViz()
cubeviz.load_data(...)

cubeviz.specviz.some_specviz_helper(...)

and that should be documented as the intended approach to using specviz helpers in the cubeviz context. But I'm not doing that now because I want to make sure others are ok with this idea before I complete this. Hence the "draft" status.

[codecov]

Codecov Report

Merging #330 (aeeb498) into main (5a7f571) will increase coverage by 1.01%.
The diff coverage is 60.00%.

❗ Current head aeeb498 differs from pull request most recent head 0f0c8ab. Consider uploading reports for the commit 0f0c8ab to get more accurate results

@@            Coverage Diff             @@
##             main     #330      +/-   ##
==========================================
+ Coverage   65.04%   66.05%   +1.01%     
==========================================
  Files          49       47       -2     
  Lines        2818     2740      -78     
==========================================
- Hits         1833     1810      -23     
+ Misses        985      930      -55     

Impacted Files	Coverage Δ
jdaviz/configs/cubeviz/helper.py	60.00% <57.14%> (-6.67%)	⬇️
jdaviz/configs/mosviz/helper.py	70.32% <62.50%> (-1.44%)	⬇️
jdaviz/core/data_formats.py
jdaviz/core/config.py
jdaviz/app.py	83.19% <0.00%> (+3.03%)	⬆️
jdaviz/core/events.py	75.48% <0.00%> (+4.51%)	⬆️
jdaviz/configs/mosviz/plugins/viewers.py	87.75% <0.00%> (+5.57%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 4ead05b...0f0c8ab. Read the comment docs.

[eteq]

After some out-of-band discussion w/ @javerbukh @dtn5ah @rosteen @havok2063 , there's some agreement that this is the way we want to go as an idiom for connecting together "sub-vizes". So I'm converting to "regular" PR.

[rosteen]

We definitely should make a note of this in the docs, and probably add a test for Mosviz and Cubeviz to check that it works (maybe just a call to e.g. viz.specviz.get_spectra() or something). Do you have bandwidth to make those additions @eteq or should someone else take over?

Edit: I should also note that I tested the actual functionality in Cubeviz and Mosviz and it works in both.

[eteq]

Hmm. I think I have time, but I have said that before and been wrong. Maybe give it another several days, @rosteen, and if I haven't gotten to it by then someone else should jump in?

[ibusko]

I tested the basic loading sequence and it works. LGTM

[rosteen]

Approving this, I have a couple docs additions that I'll put in a follow-up PR shortly, rather than adding them here. I'll resolve the merge conflicts here.

[rosteen]

Suggested two blank line addition/deletions to fix codestyle errors.