[tenpy] lift the conserve="Sz" requirement

[mrossinek]

Currently, the tenpy backends require Sz conservation. We should consider lifting this limitation.

qiskit-addon-mpf/qiskit_addon_mpf/backends/tenpy_layers/model.py

Lines 51 to 52 in 191dc81

	# TODO: currently, this default Sz conservation is coupled to the LegCharge definition in
	# MPOState.initialize_from_lattice. Not conserving Sz errors with 'Different ChargeInfo'.

Currently, the LayerModel.init_sites method defaults to conserve="Sz" for the SpinHalfSite objects being created. While one can theoretically overwrite this via the model_params, setting it to None currently breaks the leg-charges of the initial MPO state. This has to be addressed here:

qiskit-addon-mpf/qiskit_addon_mpf/backends/tenpy_tebd/state.py

Lines 70 to 71 in 191dc81

	# FIXME: the following is supposed to allow `conserve=None` for our `SpinHalfSite` but it
	# does not seem to work, even in the `conserve="Sz"` case.