Correct license information for lz4 implementation

[eborisch]

H5Zlz4.c is clearly derived from the initial HDF5 LZ4 implementation from Dectris h5zlz4.c in 2013.

I'm not sure where the history was confused along the way, but this was never a plugin implemented by h5py or the HDF5 group.

This was released under the BSD 2-clause license by Dectris in 2013. You will also find the initial filter definition document there, which would be good to include for refrence here.

See also (for history): https://forum.hdfgroup.org/t/lz4-filter-filter-id-32004/3366/3

I'll grant that it's a small piece of code, and relatively straightforward to implement, but if you're going to bother having a license file for the code, it ought to be the correct one.