Support for fetching interactions from the lower triangle when it makes sense

[robomics]

Support fetching interactions from the matrix lower triangle in the following scenarios:

• Query is materialized as a sparse matrix
• Query is materialized as a dense matrix
• Query is materialized as a table of pixels

Iterating over pixels from the lower triangle is not (and will never be) possible, as doing so would require pre-loading and re-sorting all pixels overlapping a query (defeating the purpose of iterators).

Closes #174
Closes #175

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.75000% with 15 lines in your changes are missing coverage. Please review.

Project coverage is 85.10%. Comparing base (9f426e7) to head (627bd08).
Report is 21 commits behind head on main.

Files	Patch %	Lines
...lude/hictk/transformers/impl/to_dataframe_impl.hpp	92.99%	15 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #179      +/-   ##
==========================================
+ Coverage   84.91%   85.10%   +0.18%     
==========================================
  Files         119      120       +1     
  Lines       10076    10305     +229     
==========================================
+ Hits         8556     8770     +214     
- Misses       1520     1535      +15     

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