Bug in SystemChain with sites of different Hilbert space dimensions

[gefux]

Something goes wrong when using SystemChain with sites of different Hilbert space dimensions, such as in the following example of two sites with dimensions 2 and 3:

import oqupy
import numpy as np

up_dm = oqupy.operators.spin_dm("z+")
mixed_3_dm = np.identity(3)/3.0

initial_augmented_mps = oqupy.AugmentedMPS([up_dm, mixed_3_dm])
system_chain = oqupy.SystemChain(hilbert_space_dimensions=[2,3])

pt_tebd_params = oqupy.PtTebdParameters(
    dt=0.1,
    order=2,
    epsrel=1.0e-6)

pt_tebd = oqupy.PtTebd(
    initial_augmented_mps=initial_augmented_mps,
    system_chain=system_chain,
    process_tensors=[None, None, None, None, None],
    parameters=pt_tebd_params,
    dynamics_sites=[0, 1],
    chain_control=None)

num_steps = 20
results = pt_tebd.compute(num_steps, progress_type="bar")

... fails with error message:

/data/git/OQuPy/oqupy/system.py in get_nn_full_liouvillians(self)
    666                 factor_l * np.kron(liouv_l, id_r) \
    667                 + factor_r * np.kron(id_l, liouv_r) \
--> 668                 + liouv_nn
    669 
    670             nn_full_liouvillians.append(nn_full_liouvillian)

ValueError: operands could not be broadcast together with shapes (36,36) (16,81) 

[gefux]

This has been solved with #71