clean up image parsing and tests

[cshanahan1]

1. Moved _get_data_from_image, which was previously part of _parse_image but used on its own in Background, to a method in _ImageParser instead of a function in the same module, to clean up imports.
2. Fixed some tests in test_image_parsing. Previously to test _ImageParser.parse_image, the method was being accessed through FlatTrace (which inherits _ImageParser and therefore has parse_image), and 'object' was used seemingly to avoid failing when 'self' was accessed. This was working for the time being, but will cause issues when other methods are called within _parse_image (like the one added in this PR)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (715361e) 81.77% compared to head (951646e) 81.94%.
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #210      +/-   ##
==========================================
+ Coverage   81.77%   81.94%   +0.17%     
==========================================
  Files          10       10              
  Lines        1004     1008       +4     
==========================================
+ Hits          821      826       +5     
+ Misses        183      182       -1     

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[tepickering]

looks good and cleans up the namespace. one question: if a class has a leading _, do its static and other methods need them as well? i would lean towards "no", but don't feel that strongly about it.

[cshanahan1]

if a class has a leading _, do its static and other methods need them as well? i would lean towards "no", but don't feel that strongly about it.

thats a good question, i can see the case for both ways. if the class is 'private' i guess it would be implied its methods are too, so another leading underscore would be redundant? but it also would seem logical for them to have a leading underscore as well. in another PR i can do some cleanup and make sure everything follows one convention or the other.