fix: access more frames via direct read_frame() method

[tlambert03]

the file in #190 revealed a case where sometimes there are more frames available and readable than the product of the experiment axes. This PR makes those frames accessible by slightly relaxing the logic of the reader read_frame method.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (5a7eb19) 95.40% compared to head (2289046) 95.43%.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #192      +/-   ##
==========================================
+ Coverage   95.40%   95.43%   +0.03%     
==========================================
  Files          17       17              
  Lines        2348     2346       -2     
==========================================
- Hits         2240     2239       -1     
+ Misses        108      107       -1     

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[codspeed-hq]

CodSpeed Performance Report

Merging #192 will improve performances by 34.97%

_{Comparing read_frame (2289046) with main (5a7eb19)}

Summary

⚡ 1 improvements
✅ 12 untouched benchmarks

Benchmarks breakdown

	Benchmark	main	read_frame	Change
⚡	test_time_imread[jonas_control002]	200.7 ms	148.7 ms	+34.97%