Fix RuntimeError when converting qiskit instructions.

[CalMacCQ]

Addresses #200

Consider the following example

import numpy as np
from qiskit import QuantumCircuit
from qiskit.circuit.library import RealAmplitudes
from pytket.extensions.qiskit import qiskit_to_tk

qc = QuantumCircuit(3)

params = [np.pi/2]*9
supply1 = RealAmplitudes(3, reps=2)
   
supply1 = supply1.assign_parameters(params)
qc.compose(supply1, qubits = [0, 1, 2], inplace=True)

If we try to convert qc to a pytket Circuit with qiskit_to_tk then we get

RuntimeError: Gate has an invalid number of parameters

Now pytket-qiskit converts the instruction to a CustomGate. The actual gate definition works fine but the Circuit.add_custom_gate() method is failing because there are no free symbols in the circuit.

I've decided to just handle this case with CircBox as I think its simpler.

There is still one assert failure in test which I'm puzzled by. This is the one that checks the name of the circuit.
EDIT: Resolved

[CalMacCQ]

I noticed that pylint didn't pick up this unused import in previous workflow runs.

I'd expect it to complain about this.

[cqc-alec]

For completeness can we also check the unitary against that of the original qiskit circuit?

from qiskit.quantum_info import Operator

unitary0 = Operator(qc).data

(Might have to correct for endianness?)

[CalMacCQ]

Yeah makes sense. Done.