Various improvements and bug fixes to code related to fetching interactions as Eigen matrices

[robomics]

These bugs were discovered while developing #226.

[codecov]

Codecov Report

Attention: Patch coverage is 83.49515% with 17 lines in your changes missing coverage. Please review.

Project coverage is 75.48%. Comparing base (ff82e76) to head (9875298).
Report is 35 commits behind head on main.

Files with missing lines	Patch %	Lines
.../hictk/transformers/impl/to_sparse_matrix_impl.hpp	79.48%	8 Missing ⚠️
...hic/include/hictk/hic/impl/pixel_selector_impl.hpp	0.00%	7 Missing ⚠️
...transformers/include/hictk/transformers/common.hpp	77.77%	0 Missing and 2 partials ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #227      +/-   ##
==========================================
- Coverage   75.85%   75.48%   -0.38%     
==========================================
  Files         140      141       +1     
  Lines       11580    11685     +105     
  Branches     1311     1330      +19     
==========================================
+ Hits         8784     8820      +36     
- Misses       1717     1774      +57     
- Partials     1079     1091      +12     

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