[feature request] Discover and load compression plugins

As mentioned in #2161 and https://github.com/h5py/h5py/issues/928#issuecomment-353715290, the official way for packages to support plugins is using entry points.

By officially supporting plugins that register new compression schemes, unpythonic and error prone mechanisms like hdf5plugin modifying h5py’s global variables state on import can be avoided.

hdf5plugin plugin could then register itself e.g. like this:

[project.entry-points.'h5py.compression']
lz4 = 'hdf5plugin:LZ4'
zstd = 'hdf5plugin:Zstd'

While h5py loads their projects:

from collections import ChainMap
from importlib.metadata import entry_points

FILTERS_BUILTIN = {'gzip': ..., 'lzf': ..., 'szip': ...}
FILTERS_PLUGIN = {ep.name: ep.load() for ep in entry_points(group='h5py.compression')}
FILTERS = ChainMap(FILTERS_BUILTIN, FILTERS_PLUGIN)

[vasole]

It would be so simple if the work done by hdf5plugin developers could be made by the HDF Group …

[t20100]

h5py and hdf5plugin are constrained by libhdf5 API and behaviour (as any library wrappers), so it’s not always easy/possible to be Pythonic.

Now to focus on entry point/behaviour issue, from hdf5plugin side:

• We are ready to adapt to an entry point integration if the h5py dev team is taking this way.
• To me, it is not worth breaking the current behaviour of import hdf5plugin loading HDF5 filters for e.g., import hdf5plugin; hdf5plugin.load_filters().