Implement unit conversion for contour levels

[astrofrog]

Allow the units used to show the contour levels to be customized.

Work in progress!

[codecov]

Codecov Report

Attention: Patch coverage is 49.23077% with 33 lines in your changes are missing coverage. Please review.

Project coverage is 86.12%. Comparing base (b46e487) to head (a470bc2).
Report is 2 commits behind head on main.

❗ Current head a470bc2 differs from pull request most recent head 2e0f056. Consider uploading reports for the commit 2e0f056 to get more accurate results

Files	Patch %	Lines
glue_jupyter/bqplot/image/tests/test_visual.py	23.07%	20 Missing ⚠️
glue_jupyter/bqplot/image/state.py	59.37%	13 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #423      +/-   ##
==========================================
- Coverage   86.60%   86.12%   -0.49%     
==========================================
  Files          89       90       +1     
  Lines        5167     5226      +59     
==========================================
+ Hits         4475     4501      +26     
- Misses        692      725      +33     

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[javerbukh]

Jdaviz implementation of contour unit conversion: spacetelescope/jdaviz#2702

[astrofrog]

Looks like the CircleCI test failure is real, will try and take a look tomorrow morning.