Implement driver that emits user-defined layer hierarchy

[knoepfel]

A user should be able to specify something like:

driver : {
  plugin: "layer_generator",
  layers: {
    run: { number: 3 },
    subrun: { parent: "run", number: 10 },
    spill: { parent: "subrun", number: 10 },
    calib: { number: 4},
},

which is an indication that 3 run data cells will be created, each with 10 subrun data cells, with each subrun data cell containing 10 spill data cells. In addition, each job has 4 calibration data cells. The total number of processed data cells being:

Runs:     3
Subruns: 30
Spills: 300
Calib:    4