switch to zstd for compression

[orgua]

pro

• should have less cpu-overhead (can't be low enough on that old BBone-cpu)
• and less io-calls / better compression (which also results in less cpu-overhead on BBone)

contra

• not out-of-the-box supported by hdf5 (so other tools might not be able to open shepherd-files)
• needs another python-package hdf5plugin.Zstd()

example-code

[orgua]

pip install hdf5plugin

begins to compiling forever on BBone and apt offers no ready-to-use package. so there is currently no smooth way to test zstandard.

[orgua]

abandoned in favor of #59