Restructuring of HDF5 output to reduce file size. This
required the simplification of the internal file group structure
and the stacking of datasets to allow efficient compression and
avoid extra overhead of storing links to many datasets.

Only one folder is saved per pixel now, and each folder contains:
    * los (line-of-sight extinction profile)
    * stellar chains
    * stellar pdfs

Without the stellar pdfs, the expected file size per star is
about 2 kB. With the stellar pdfs, and after applying compression,
the expected file size is about 25 kB.