Support variable bin sizes

[robomics]

• Rename BinTable to BinTableFixed
• Implement BinTableVariable to support operating on tables with variable bin size
• BinTable is now a wrapper around BinTableFixed and BinTableVariable
• Update Reference to store the prefix sum of the chromosome sizes
• Fix minor bug in Reference ctor when constructing reference from iterators of chromosomes
• Update hictk load to support loading interactions using tables of variable bin size
• Update hictk merge to support merging interactions using tables of variable bin size

[codecov]

Codecov Report

Attention: 75 lines in your changes are missing coverage. Please review.

Comparison is base (d9727c0) 83.65% compared to head (80965a7) 83.92%.
Report is 5 commits behind head on main.

Files	Patch %	Lines
...ble/include/hictk/impl/bin_table_variable_impl.hpp	87.83%	23 Missing ⚠️
..._table/include/hictk/impl/bin_table_fixed_impl.hpp	92.14%	15 Missing ⚠️
src/hictk/cli/cli_load.cpp	68.42%	6 Missing ⚠️
...include/hictk/cooler/impl/file_validation_impl.hpp	0.00%	4 Missing ⚠️
...ler/include/hictk/cooler/impl/utils_merge_impl.hpp	50.00%	4 Missing ⚠️
...erval/include/hictk/impl/genomic_interval_impl.hpp	88.23%	4 Missing ⚠️
src/hictk/cli/cli_zoomify.cpp	25.00%	3 Missing ⚠️
src/hictk/load/load.cpp	94.33%	3 Missing ⚠️
...oler/include/hictk/cooler/impl/validation_impl.hpp	0.00%	3 Missing ⚠️
src/hictk/convert/cool_to_hic.cpp	33.33%	2 Missing ⚠️
... and 5 more

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #81      +/-   ##
==========================================
+ Coverage   83.65%   83.92%   +0.27%     
==========================================
  Files          91       96       +5     
  Lines        6742     7111     +369     
==========================================
+ Hits         5640     5968     +328     
- Misses       1102     1143      +41     

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