DensityMatrix snapshot extension

[chriseclectic]

What is the expected behavior?

This will be used by the density matrix method of the QasmSimulator.

class SnapshotDensityMatrix(Snapshot):
    __init__(self, label, num_qubits):
        super(label, "density_matrix", num_qubits, 0, None)
 

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

circuit.snapshot_density_matrix = snapshot_density_matrix

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.