[Bug]: Incorrect number of qubits in statevector simulator

[matulni]

The property nqubit of Statevec objects fails to return the correct number of qubits. See examples attached:

from graphix import Pattern

p = Pattern(input_nodes=[0, 1])
sim_py = p.simulate_pattern()
assert sim_py.nqubit == 2  # Error: sim_py.nqubit -> 3

from graphix.sim.statevec import Statevec
sv = Statevec(nqubit=3)
assert sv.nqubit == 3  # Error: sv.nqubit -> 7

DensityMatrix objects return the correct values:

from graphix import Pattern

p = Pattern(input_nodes=[0, 1])
sim_py = p.simulate_pattern(backend='densitymatrix')
print(sim_py.nqubit)
assert sim_py.nqubit == 2

from graphix.sim.density_matrix import DensityMatrix
sv = DensityMatrix(nqubit=3)
assert sv.nqubit == 3