[T.1.1] Add Support for Custom Data Sources and Provenance Tracking

[smarr]

It would be useful to be able to track custom metrics, for instance:

• [ ] hardware counters
• [ ] memory usage
• [x] specific parts of a program's execution
• [ ] any kind of custom metrics
• [ ] single value metrics for builds, run jobs (overall time, binary size, ...)

ReBench can already capture such data from a single data source, it does not yet support reading data from different input sources, and it doesn't yet have a mechanism to get single values, from the "build", which are not necessarily associated with a specific run.

Having different data sources/files is pretty common, especially when using tools to measure metrics.

The challenge is to keep simple things simple, and make complex customization require less effort than writing a custom tool. This is key to make ReBench useful for a wider community.

This needs to be applied to the tracking of custom provenance information, too. For instance, tracking of GPU drivers versions, processor temperature, and CPU microcode revisions.