`gen_UnCorrelated_SPAM_MitRes` hangs on larger architectures

[sjdilkes]

Failing code snippet:

from pytket.extensions.qiskit import IBMQEmulatorBackend

casablanca_backend = IBMQEmulatorBackend(
   "ibmq_toronto",
   hub='',
   group='',
   project='',
)
uc_spam_mitres = gen_UnCorrelated_SPAM_MitRes(
   backend = casablanca_backend,
   calibration_shots = 500
)
from pytket import Circuit
from qermit import CircuitShots

test_c_0 = Circuit(4).X(0).X(2).measure_all()

test_experiment = [CircuitShots(Circuit = test_c_0, Shots = 1000)]
res = uc_spam_mitres.run(test_experiment)

[sjdilkes]

The method does not hang, the simulator just takes an extremely long amount of time to perform 2^27x2^27 matrix multiplications.