Create driver interface for a fixed-hierarchy job

[knoepfel]

We need an interface to support user-defined drivers for jobs with fixed hierarchies (e.g.):

void cells_to_process(framework_driver& driver) { ... }

PHLEX_REGISTER_DRIVER(d, config)
{
  //  We want to express a hierarchy that looks like:
  // 
  //    run
  //     │
  //     ├ subrun
  //     │  │  
  //     │  └ spill
  //     │
  //     └ calibration_period
  //
  //  This is done by specifying the hierarchy "paths".
 
  d.driver(phlex::fixed_hierarchy{{"run", "subrun", "spill"},
                                  {"run", "calibration_period"}}, 
           cells_to_process);
}