Fix lindbladian

[Ashutosh-Mishra2]

What

Fixes Lindblad evolution in propagation. Also, changed the batch propagation to make batches of time slices. This helps with large simulations to use less RAM while doing propagation.

Why

Previous propagation code was missing the lindblad evolution and used to return unitary matrices instead of superoperators. Closes #215

How

Added a condition if model.lindbaldian is True then use tf_propagation_lind for calculating the superoperators for evolution. Also changed signals[i * batch_size : i * batch_size + batch_size] to signals[:, i * batch_size : i * batch_size + batch_size] to make batches of time slices for propagation instead of batches of control lines.

Remarks

Changing the way batches are made means that one has to provide the batch size for the simulation by using exp.propagate_batch_size. The default is set to be the length of signal, so it creates only one batch.

Checklist

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