Implement robust provenance and reproducibility

[dkirkby]

A config file fully specifies a simulation at the time it is used, but simulation parameters are typically changed on the fly (e.g., airmass, exposure time, source spectrum) so the input config file is not sufficient for specifying exactly how a simulation was run.

This issue is to develop a plan for providing more robust log of exactly how a simulation was performed with, ideally, the ability to reproduce a simulation after the fact.

This is not urgent, but something to start thinking about.