Updated Zstd plugin; Added support for negative compression levels to Zstd

[t20100]

! Merge PR #394 first !

This PR changes upstream project for the Zstd filter from the initial repository (ttps://github.com/aparamon/HDF5Plugin-Zstandard) to the version hosted by the hdfgroup organisation (https://github.com/HDFGroup/hdf5_plugins).

The reason is that the former project is stalled while the later is actively maintained.
For instance, support for negative compression levels (favor speed over compression ratio) as initially proposed in #165 is implemented in the hdfgroup's version of the filter.

This PR also adds support for negative compression levels in hdf5plugin.Zstd class.

[t20100]

Need to pass uint32. The filter cast this back to int

[payno]

And we are agree that it is cast the same way ?

clevel = -12
clevel_uint32 = struct.unpack("I", struct.pack("i", clevel))[0]
struct.unpack("i", struct.pack("I", clevel_uint32))[0]
>> -12

Could we add a comment on this. Because indeed this is a bit tricky and better to highlight that this is done on purpose...

[t20100]

I hesitated to make helper functions to make it more explicit.

[payno]

To be honest I saw the reverse function a couple of line after but I let the comment.

This is super simple I agree but I would be in favor of helper function (This is easier to test and to associate).

[t20100]

Added _to_uint32 and _from_uint32 helpers in 58afed3

[payno]

And we are agree that it is cast the same way ?

clevel = -12
clevel_uint32 = struct.unpack("I", struct.pack("i", clevel))[0]
struct.unpack("i", struct.pack("I", clevel_uint32))[0]
>> -12

Could we add a comment on this. Because indeed this is a bit tricky and better to highlight that this is done on purpose...