ssx_index, ssx_integrate cleanup

[jbeilstenedmands]

Remove overly complicated handling of multiple imagesets.

Avoid iterating over null tables and slow insertions for significant performance gains for the case of large 'sparse' datasets.

Improve code structure for reusability.

[codecov]

Codecov Report

Merging #2169 (c220eea) into main (3d995d5) will increase coverage by 0.03%.
The diff coverage is 90.23%.

❗ Current head c220eea differs from pull request most recent head 12df945. Consider uploading reports for the commit 12df945 to get more accurate results

@@            Coverage Diff             @@
##             main    #2169      +/-   ##
==========================================
+ Coverage   80.58%   80.61%   +0.03%     
==========================================
  Files         585      587       +2     
  Lines       66216    66449     +233     
  Branches     9342     9360      +18     
==========================================
+ Hits        53360    53571     +211     
- Misses      10805    10807       +2     
- Partials     2051     2071      +20