Statevector snapshot extension

[chriseclectic]

What is the expected behavior?

This should have signature

class SnapshotStatevector(Snapshot):
    __init__(self, label, num_qubits):
        super(label, "statevector", num_qubits, 0, None)
 

# Monkey patch function
def snapshot_statevector(self, label, qubits=None):
     # see base snapshot monkey patch for converting qubits to
     # be all qubits in circuit

circuit.snapshot_statevector = snapshot_statevector

The num_qubits parameter is actually ignored by Aer (it is always an all-qubit instruction), however it is used by Terra to make the snapshot act as a barrier.