Implement bins_ptr() accessor for cooler::SingleCellFile()

[robomics]

This should've been part of #199.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.56%. Comparing base (70a1d2d) to head (1e20e03).
Report is 10 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #298      +/-   ##
==========================================
- Coverage   77.58%   77.56%   -0.02%     
==========================================
  Files         173      173              
  Lines       21222    21225       +3     
  Branches     2310     2310              
==========================================
- Hits        16465    16464       -1     
  Misses       3755     3755              
- Partials     1002     1006       +4     

Flag	Coverage Δ
[tests	integration](https://app.codecov.io/gh/paulsengroup/hictk/pull/298/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=paulsengroup)	68.01% <100.00%> (-0.01%)
[tests	unittests](https://app.codecov.io/gh/paulsengroup/hictk/pull/298/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=paulsengroup)	75.92% <100.00%> (+0.07%)

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