Handle register conditions/multiple bits for IfElseOp in qiskit_to_tk

[CalMacCQ]

After #437 we can now convert circuits containing IfElseOp instructions to equivalent pytket circuits with qiskit_to_tk

Currently the conversion is limited to conditions on single bits and gives a NotImplemtedError when we try to convert an If-else conditioned on a register.

It shouldn't be much work to remove the restriction to a single bits and convert a wider range of operations

Example (taken from the tests)

from qiskit import QuantumCircuit
from qiskit import QuantumRegister, ClassicalRegister

qreg = QuantumRegister(2, "q")
creg = ClassicalRegister(2, "c")
circuit = QuantumCircuit(creg, qreg)
(q0, q1) = qreg
(c0, c1) = creg

circuit.h(q0)
circuit.h(q1)
circuit.measure(q0, c0)
circuit.measure(q1, c1)
# Condition is on a register not a bit
with circuit.if_test((creg, 2)):
    circuit.x(q0)
    circuit.x(q1)
circuit.measure(q0, c0)
circuit.measure(q1, c1)

[cqc-alec]

This is kinda blocking #438 since it's needed to make round-trip tests work once we use if_test instead of c_if in tk_to_qiskit().

[CalMacCQ]

Good spot, probably worth prioritising then