add optional labels to standard gates

[chriseclectic]

Summary

• Fix rounder error for using atol parameter in truncating probabilities in QuantumError (closes Composing a depolarizing error with a thermal error fails for certain values #176).
• Composing errors with unitary or Kraus instructions now fuses additional instructions.
• Adds optional labels to all standard gate operations. This allows adding a noise model to specially labeled gates as with the unitary instruction in the noise model.
• If a label isn't specified the gate name is used as the label so there is no backwards compatibility.

NOTE: this isn't supported through Terra circuits yet, this is to preemptively to add support to Aer from the qobj level.

Details and comments

If a qobj circuit had instructions: [{"name": "id", "qubits": [0]}, {"name": "id", "qubits": [0], "label": "alt_id"}] then adding a noise model with quantum error error_a to "id" and error_b to "alt_id" would sample from the two different errors for each of the identity instructions.

Example

from qiskit import *
from qiskit.providers.aer import QasmSimulator, noise

qr = QuantumRegister(2)
cr = ClassicalRegister(2)
circ = QuantumCircuit(qr, cr)
circ.iden(qr[0])
circ.iden(qr[1])
circ.barrier(qr)
circ.measure(qr, cr)

qobj = assemble_circuits(circ)
# edit qobj
qobj.experiments[0].instructions[1].label = 'id1'
qobj.config.shots = 10000

# Ideal
result = QasmSimulator().run(qobj).result()
print(result.get_counts(0))  # {'00': 10000}

# noise model
error0 = noise.errors.depolarizing_error(0.25, 1)
error1 = noise.errors.depolarizing_error(0.5, 1)
noise_model = noise.NoiseModel()
noise_model.add_all_qubit_quantum_error(error0, 'id')
noise_model.add_all_qubit_quantum_error(error1, 'id1')

result = QasmSimulator().run(qobj, noise_model=noise_model).result()
print(result.get_counts(0))  # {'11': 323, '00': 6532, '01': 943, '10': 2202}