Fletcher checksum codec

[rabernat]

It would be great if numcodecs could implement the Fletcher checksum algorithm in a way compatible with how hdf5 does it. In this version, the checksum is simply appended to the end of the chunk bytes. See https://github.com/fsspec/kerchunk/pull/274 for a dummy passthrough version.

There are some python implementations here:

• https://github.com/njaladan/hashpy/blob/master/hashpy/fletcherNbit.py
• https://gist.github.com/globby/9337839
• https://gist.github.com/AJPoulter-Soton/9d0d2505af64f0719bdee59b9a4533ba

[martindurant]

Some (fast) C implementations mentioned in https://stackoverflow.com/questions/40270450/correctness-of-fletcher32-checksum-algorithm (would cython well)

[rabernat]

And just for completeness, here is the hdf5 version as vendored by netcdf4:

https://github.com/Unidata/netcdf-c/blob/8eb71290eb9360dcfd4955ba94759ba8d02c40a9/plugins/H5checksum.c