Support custom multiqubit operations in simulator

[thisac]

Only a subset of multi-qubit gates are supported in the simulator, causing custom multi-qubit operations to fail.

bell = Circuit(2)

with bell.context as (q, c):
    ops.Hadamard(q[0])
    ops.CNOT(q[0], q[1])

Bell = create_operation(bell, "Bell")

circuit = Circuit(2)

with circuit.context as (q, c):
    Bell(q)

raises

simulator encountered unknown multi-qubit operation: Bell

Two potential solutions are:

• Add support for arbitrary unitaries in the simulator.
• Automatically decompose custom gates before running them on the simulator.