[WIP] Commutation function and commutation DAG

[rmoyard]

Context:

This PR adds a commutation (pairwise) representation of quantum circuits. This is useful for applying different optimization algorithms (template matching, peephole optimization) that need commutation structure of the circuits.

Paper More details about the DAG in section 2.2

Description of the Change:

This PR adds a

1. Function is_commuting checks if two operations commute, it does not use matrices multiplications but a truth table. Each operations has now attributes is_controlled and target_operation. Each operation has also functions that return control wires and target wires control_wires() target_wires(). Those new attributes and functions are useful for determining commutation between operations.
2. [WIP] A DAG that represents the pairwise commutation structure of the quantum circuit.

x = np.array([0.1, 0.2])

dev = qml.device('default.qubit', wires=2)
@qml.qnode(dev)
def circuit(x):
    qml.RX(x[0], wires=0)
    qml.CNOT(wires=(0,1))
    qml.RX(x[1], wires=0)
    return qml.probs(wires=(0,1))

qml.commutation_dag(circuit)(x)

Benefits:
A commutation function that does not use matrices multiplication and takes any two operations as inputs. A new DAG that represents the commutation structure of quantum circuits and is useful for quantum circuits optimization.