[bug]: fix handling of Cooler v2 files lacking the "storage-mode" attribute

[robomics]

According to Cooler docs:

Version 3 introduces the storage-mode metadata attribute to accomodate square matrices that are non-symmetric. Version 2 files which lack the storage-mode attribute should be interpreted as using the “symmetric-upper” storage mode. See Storage mode.

Given this, the check we had in place was too strict.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.35%. Comparing base (2bd1af2) to head (1a444c7).
Report is 2 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #339      +/-   ##
==========================================
+ Coverage   77.31%   77.35%   +0.03%     
==========================================
  Files         175      175              
  Lines       21436    21440       +4     
  Branches     2326     2327       +1     
==========================================
+ Hits        16574    16584      +10     
+ Misses       3848     3843       -5     
+ Partials     1014     1013       -1     

Flag	Coverage Δ
[tests	integration](https://app.codecov.io/gh/paulsengroup/hictk/pull/339/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=paulsengroup)	67.85% <100.00%> (+0.01%)
[tests	unittests](https://app.codecov.io/gh/paulsengroup/hictk/pull/339/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=paulsengroup)	75.81% <0.00%> (-0.02%)

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