perf: hipo2npz memory efficiency

[c-dilks]

The current hipo2npz is not memory efficient, since it effectively reads and stores a full HIPO files' data structures in memory while producing the NPZ file.

This PR replaces the memory storage with disk storage, by creating a directory of temporary files in <output>.tmp/, where <output> is the output file name. Now the memory usage hovers under 500 MB typically for a DST file.

I had Claude generate an NPZ diff tool, stored here as hipo2npz-diff; I used it to verify there is no difference in the resulting NPZ file before and after this PR.

About Temporary Files

• We do not use the /tmp file system, to keep the temp files and output NPZ file on the same disk.
• The temporary files are deleted when:
  • the conversion is done
  • anything that "nicely" kills the job, such as SIGINT or SIGTERM
• The temporary files remain if the job is not-so-nicely killed (SIGKILL), since the JVM does not have a chance to remove them in that case; this means if we run on SLURM, it may be possible that OOM or time-limit kills from SLURM will leave temp files.
• If a directory, file, or symlink with the name <output>.tmp already exists, hipo2npz will immediately fail, preventing accidental deletion of something that was not created by hipo2npz

Additional changes

• new options --num-events and --first-event to convert a range of events

[c-dilks]

Oops. I need to revert this when the host is back online.