Produce cloud-optimized ATL03 HDF5 example files

Beyond increasing chunk size:

- Minimal use of variable-length datatypes
- Consolidated internal file metadata
- HDF5 paged aggregation (recommended if HDF5 library will also be used for cloud access)

See https://github.com/HDFGroup/nasa_cloud/blob/main/benchmarks/python/icesat2_selection.py - we may be able to leverage this work. 