Imviz regions setter/getter API

[pllim]

Fix #622. Second attempt of #630. (It is easier to open a new PR than to resolve conflicts of that one...)

Note to self: Open follow-up issues about modifying regions, roundtripping, GWCS support...

[codecov]

Codecov Report

Merging #721 (650d13c) into main (e4b61d2) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main     #721      +/-   ##
==========================================
+ Coverage   62.40%   62.52%   +0.12%     
==========================================
  Files          65       65              
  Lines        4237     4267      +30     
==========================================
+ Hits         2644     2668      +24     
- Misses       1593     1599       +6     

Impacted Files	Coverage Δ
jdaviz/app.py	79.30% <100.00%> (+0.10%)	⬆️
jdaviz/configs/imviz/helper.py	98.99% <100.00%> (+0.16%)	⬆️
...configs/default/plugins/data_tools/file_chooser.py	65.71% <0.00%> (-3.43%)	⬇️

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[pllim]

So, using "reference data" got around the problem I encountered in #630 . 😸

[rosteen]

Looks good, I tested the code in the example notebook and it works. The API makes sense to me. I noted some performance issues in #723, I don't think they need to hold up this ticket but should be a priority to fix, in my opinion.

[ibusko]

Code looks good. I tested with the example notebook and it works as expected.

[pllim]

Given there are 2 approvals, I am merging. Thanks, everyone!

p.s. I ran it locally after rebase and nothing broke. The lag is very apparent but out of scope, as Ricky said.