fix(pqread.py): allow subdivision1 filtering

[bassosimone]

This diff adds support for filtering for subdivision1 and extends the tests to ensure we're testing for it.

While there, notice that the test suite was not adequately testing filtering by columns (i.e., ensuring that we can get the columns we want), thus introduce a fixture file containing expected columns, to improve the tests quality.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00%. Comparing base (4ba6f88) to head (3f3e453).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##              main       #93   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           27        27           
  Lines         1838      1853   +15     
=========================================
+ Hits          1838      1853   +15     

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