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

Before you get started

• ☝ Create an issue to discuss what you are going to do

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