Add some missing snapshots

[chriseclectic]

Summary

Adds three new snapshots:

1. "stabilizer" snapshot for QasmSimulator stabilizer simulation method. This returns the current stabilizer state for each shot
2. "expectation_value_pauli_single_shot": per shot expectation values of Pauli matrix (instead of averaged over all shots)
3. "expectation_value_matrix_single_shot": per shot expectation values of matrix (instead of averaged over all shots)

Details and comments

The single shot expectation value take the same params as the other expectation value snapshots, but instead of returning the average value over shots they return the list of expectation values for each shot. These kind of snapshots are needed for quantum error correction simulations.

They are returned under the field "expectation_values" rather than "expectation_value".

Example

from qiskit import *
from qiskit.extensions.simulator import *
from qiskit.providers.aer import *

sim = Aer.get_backend("qasm_simulator")

exp_x = [np.array([[1, 0], 'X'])]
snap_ave = Snapshot('<X>_ave', 'expectation_value_pauli', 1, 0, exp_x)
snap_shot = Snapshot('<X>_shot', 'expectation_value_pauli_single_shot', 1, 0, exp_x)

nm = noise.NoiseModel()
nm.add_all_qubit_quantum_error(noise.errors.depolarizing_error(0.5, 1), 'h')

qc = QuantumCircuit(1, 1)
qc.h(0)
qc.append(snap_ave, [0])
qc.append(snap_shot, [0])
qc.measure(0, 0)
job = execute(qc, sim, shots=10, noise_model=nm)
result = job.result()
result.data(0)

Returns

{'snapshots': {'expectation_values': {'<X>_shot': [[1.0, 0.0],
    [-1.0, 0.0],
    [1.0, 0.0],
    [-1.0, 0.0],
    [1.0, 0.0],
    [1.0, 0.0],
    [1.0, 0.0],
    [1.0, 0.0],
    [1.0, 0.0],
    [1.0, 0.0]]},
  'expectation_value': {'<X>_ave': [{'memory': '0x0', 'value': [0.6, 0.0]}]}},
 'counts': {'0x1': 4, '0x0': 6}}

​

[atilag]

LGTM!
Left a simple comment that would be great if we can address.

[atilag]

What is the last true and false argument? I'd definitely put an enum here so I don't have to go the function definition to know what's this boolean means.

[atilag]

hmmm no tests?

[chriseclectic]

For tests we really need the instructions @kanejess and @merav-aharoni are working on in #300