fix fletcher32, add tests

[kmuehlbauer]

Description

This fixes the current implementation of the fletcher32 algorithm. The current testsuite didn't catch it, I only found it by accident while running the xarray testsuite. It therefore also adds a comprehensive test matrix to run over different filter pipelines including chunking, compression, fletcher32 and shuffle.

Closes #128

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Checklist

• This pull request has a descriptive title and labels
• This pull request has a minimal description (most was discussed in the issue, but a two-liner description is still desirable)
• Unit tests have been added (if codecov test fails)
• Any changed dependencies have been added or removed correctly (if need be)
• If you are working on the documentation, please ensure the current build passes
• All tests pass

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 74.03%. Comparing base (08637c3) to head (958e935).
⚠️ Report is 159 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #129      +/-   ##
==========================================
- Coverage   74.05%   74.03%   -0.02%     
==========================================
  Files          11       11              
  Lines        2598     2596       -2     
  Branches      406      406              
==========================================
- Hits         1924     1922       -2     
  Misses        565      565              
  Partials      109      109              

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[kmuehlbauer]

Great, this worked. It took a while to bubble up from xarray over h5netcdf to pyfive. And then, such nasty things going on while mixing native Python int and numpy in arithmetic.

[kmuehlbauer]

Ready for review!

[kmuehlbauer]

@bnlawrence It would be great, if you could have a look at these three lines of code. Thanks!

[kmuehlbauer]

overridde by #133, branching issue