add filters subpackage containing wheel filters

[jkotan]

It would be good to add to the pninexus wheel external filters. We can do it as a package_data of pninexus.filters package.
Importing

from pninexus import filters

would set the proper HDF5_PLUGIN_PATH.

To create the wheel we ca e.g.

export HDF5_PLUGIN_PATH=/usr/lib/x86_64-linux-gnu/hdf5/serial/plugins
export PNINEXUS_FILTERS=libh5blosc.so,libh5bshuf.so,libh5bz2.so,libh5lz4.so,libh5lzf.so,libh5mafisc.so,libh5zfp.so
python setup.py bdist_wheel
auditwheel repair dist/pninexus-3.2-cp39-cp39-linux_x86_64.whl --plat manylinux_2_31_x86_64