Simplify pulse sequence instantiation

[thangleiter]

As far as I can tell, the optimizer only updates the control amplitudes of the solver object. In that case it's much simpler to just manually set the new amplitudes if a solver.pulse_sequence already exists. When updating, we just need to make sure to clear all cached attributes (like the eigendecomposition) of the PulseSequence object.

Furthermore, the PulseSequence constructor can handle qopt.DenseOperators and the instantiation of the object in solver.create_pulse_sequence() can be simplified a bit (once qutech/filter_functions#53 is merged).

With these changes, solver.plot_bloch_sphere can also be a bit leaner and just call the solver.create_pulse_sequence() method.

[JDTeske]

The new instantiation of pulse sequences seem to treat h_drift and h_control the same way.

This will lead to inconsistencies because h_drift has one dimension less than h_control.
h_drift -> shape: (n_time)
h_ctrl -> shape: (n_ctrl, n_time)

we also only need a single label for the single drift

[thangleiter]

This will lead to inconsistencies because h_drift has one dimension less than h_control.
h_drift -> shape: (n_time)
h_ctrl -> shape: (n_ctrl, n_time)

Ah, I see. I thought you could pass any number of drift operators. However, in the current implementation one also cannot use the 'one drift operator per transferred time step' option as only the first one is used for the PulseSequence:

qopt/qopt/solver_algorithms.py

Lines 84 to 86 in 77540f3

	h_drift: List[ControlMatrix], len num_t or 1
	Drift operators in the Hamiltonian. You can either give a single element
	or one for each transferred time step.

qopt/qopt/solver_algorithms.py

Lines 633 to 636 in 77540f3

	for drift_operator in [self.h_drift[0], ]:
	if type(drift_operator) == matrix.DenseOperator:
	drift_operator = drift_operator.data
	h_c += [[drift_operator, len(self.transferred_time) * [1], 'Drift'], ]

Maybe there should be a note regarding that somewhere? I don't immediately see a way to make that option work with the PulseSequence constructor.