Add more options for HDF5 witer

This adds some options to configure settings that were previously hardcoded in the HDF5 writer:

• The chunk size can be set explicitly (rather than defaulting to the values of the N+1 dataset extents), useful for something @jburzyns was doing
• The deflate level can be set, see atlas-flavor-tagging-tools/training-dataset-dumper#123

These options can be passed to a new writer constructor. The previous constructor remains unchanged for backward compatibility.