Number of bits not preserved in tk_to_qiskit conversion with RangePredicate

[CalMacCQ]

Whilst looking into #375 I found that the number of bits was not preserved in the tk_to_qiskit conversion. I think this may well be the cause of the KeyError issue.

from pytket import Circuit, OpType
from pytket.circuit.logic_exp import reg_eq
from pytket.extensions.qiskit import tk_to_qiskit


main_circ = Circuit(2, 2, "main_circ")

creg_0 = main_circ.add_c_register("creg_0", 2)

main_circ.add_gate(OpType.CH,[0, 1], condition = reg_eq(creg_0, 3))

main_circ.measure_all()
print(main_circ.n_bits) # gives 5

But if I convert to a qiskit QuantumCircuit...

qc = tk_to_qiskit(main_circ)
print(qc.num_clbits) # gives 4

We only get 4 bits in the converted qiskit circuit. it seems the tk_SCRATCH_BIT[0] bit is lost in the conversion.