Write a JSON metadata file summarizing the inputs to a run and its output files. This file provides ParaView and other post-processing tools a simple way to aggregate data for visualizations and analyses.

* Write a metadata file as the simulation is run.

Metadata is collected during simulation startup and updated to
include timestep information with each step the simulation takes.
It is rewritten with each timestep so that separate processes
may observe simulation progress by watching the file for changes.

* Properly pfundist specific storage.