Add dependency graph infrastructure

[shwestrick]

We should incorporate Colin's dependency graph tool, for gate scheduling.

The script is currently available here. Usage: $ python3 depgraph.py circuit.qasm

@colin-mcd, can you add your tool to this repo?

dependency graph representation

Colin's script outputs JSON describing the dependencies between gates, for example:

{ 
  "nodes": [
    {"name": "x", "qargs": [0]},      // gate 0
    {"name": "cx", "qargs": [1, 0]},  // gate 1
    {"name": "x", "qargs": [0]},      // etc...
    {"name": "h", "qargs": [1]},
    {"name": "rz", "params": [3.141592653589793], "qargs": [1]}
  ],

  "edges": [
    [3, 4],   // gate 3 has to be executed before gate 4
    [1, 3]    // gate 1 has to be executed before gate 3
  ]
}

For the simulators, we'll just need to parse this JSON as input.

[colin-mcd]

Implemented in PR #21