Improve support for "square" Cooler files

[robomics]

See #225
Closes #225

[codecov]

Codecov Report

Attention: Patch coverage is 37.09677% with 78 lines in your changes missing coverage. Please review.

Project coverage is 74.24%. Comparing base (52bd468) to head (f59bbf0).

Files with missing lines	Patch %	Lines
...hictk/file/include/hictk/impl/utils_merge_impl.hpp	0.00%	43 Missing ⚠️
src/hictk/validate/validate.cpp	41.66%	7 Missing ⚠️
...ooler/include/hictk/cooler/impl/file_read_impl.hpp	75.00%	7 Missing ⚠️
src/hictk/cli/cli_convert.cpp	25.00%	6 Missing ⚠️
src/hictk/cli/cli_zoomify.cpp	0.00%	3 Missing and 2 partials ⚠️
...ler/include/hictk/cooler/impl/utils_merge_impl.hpp	25.00%	3 Missing ⚠️
...oler/include/hictk/cooler/impl/validation_impl.hpp	40.00%	3 Missing ⚠️
src/hictk/cli/cli_fix_mcool.cpp	50.00%	2 Missing ⚠️
.../include/hictk/cooler/impl/pixel_selector_impl.hpp	84.61%	1 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #249      +/-   ##
==========================================
- Coverage   74.48%   74.24%   -0.25%     
==========================================
  Files         144      144              
  Lines       12479    12524      +45     
  Branches     1445     1450       +5     
==========================================
+ Hits         9295     9298       +3     
- Misses       1991     2031      +40     
- Partials     1193     1195       +2     

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[robomics]

The integration tests are expected to fail because the test dataset used by this branch contains the updated files required by #244.

It is not worth creating a temporary test dataset just for this branch.
Once #249 and #244 are both merged, the tests should (hopefully!) pass.